It appears impossible to submit an invoice from M2 admin. I get the error message: “X We can’t save the invoice right now.”.

I checked the exception log and I see below errors:

    [2018-02-15 13:44:02] main.CRITICAL: Exception: Warning: Illegal string offset 'ID' in /home/my_username/domains/my_domain.com.nl/public_html/lib/mpdf/classes/cssmgr.php on line 1302 in /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/ErrorHandler.php:61 Stack trace: #0 /home/my_username/domains/my_domain.com.nl/public_html/lib/mpdf/classes/cssmgr.php(1302): Magento\Framework\App\ErrorHandler->handler(2, 'Illegal string ...', '/home/my_username/...', 1302, Array) #1 /home/my_username/domains/my_domain.com.nl/public_html/lib/mpdf/mpdf.php(15961): cssmgr->MergeCSS('BLOCK', 'BODY', '') #2 /home/my_username/domains/my_domain.com.nl/public_html/lib/mpdf/mpdf.php(31556): mPDF->WriteHTML('<img src='http:...') #3 /home/my_username/domains/my_domain.com.nl/public_html/app/code/Cis/Pdfpackage/Model/Observer/OrderStatusChanged.php(237): pdfGenerate('<img src='http:...', 'var/pdfs/000000...') #4 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Event/Invoker/InvokerDefault.php(73): Cis\Pdfpackage\Model\Observer\OrderStatusChanged->execute(Object(Magento\Framework\Event\Observer)) #5 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Event/Invoker/InvokerDefault.php(61): Magento\Framework\Event\Invoker\InvokerDefault->_callObserverMethod(Object(Cis\Pdfpackage\Model\Observer\OrderStatusChanged), Object(Magento\Framework\Event\Observer)) #6 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Event/Manager.php(66): Magento\Framework\Event\Invoker\InvokerDefault->dispatch(Array, Object(Magento\Framework\Event\Observer)) #7 /home/my_username/domains/my_domain.com.nl/public_html/var/generation/Magento/Framework/Event/Manager/Proxy.php(95): Magento\Framework\Event\Manager->dispatch('sales_order_sav...', Array) #8 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Model/AbstractModel.php(802): Magento\Framework\Event\Manager\Proxy->dispatch('sales_order_sav...', Array) #9 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Model/ResourceModel/Db/VersionControl/AbstractDb.php(56): Magento\Framework\Model\AbstractModel->afterSave() #10 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Model/ResourceModel/Db/AbstractDb.php(408): Magento\Framework\Model\ResourceModel\Db\VersionControl\AbstractDb->processAfterSaves(Object(Magento\Sales\Model\Order)) #11 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-sales/Model/ResourceModel/Order.php(174): Magento\Framework\Model\ResourceModel\Db\AbstractDb->save(Object(Magento\Sales\Model\Order)) #12 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(74): Magento\Sales\Model\ResourceModel\Order->save(Object(Magento\Sales\Model\Order)) #13 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(70): Magento\Sales\Model\ResourceModel\Order\Interceptor->___callParent('save', Array) #14 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Framework\Interception\Chain\Chain->invokeNext('Magento\Sales\M...', 'save', Object(Magento\Sales\Model\ResourceModel\Order\Interceptor), Array, 'clean_cache') #15 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/Cache/FlushCacheByTags.php(71): Magento\Sales\Model\ResourceModel\Order\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Sales\Model\Order)) #16 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(142): Magento\Framework\App\Cache\FlushCacheByTags->aroundSave(Object(Magento\Sales\Model\ResourceModel\Order\Interceptor), Object(Closure), Object(Magento\Sales\Model\Order)) #17 /home/my_username/domains/my_domain.com.nl/public_html/var/generation/Magento/Sales/Model/ResourceModel/Order/Interceptor.php(39): Magento\Sales\Model\ResourceModel\Order\Interceptor->___callPlugins('save', Array, Array) #18 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Model/AbstractModel.php(631): Magento\Sales\Model\ResourceModel\Order\Interceptor->save(Object(Magento\Sales\Model\Order)) #19 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/DB/Transaction.php(129): Magento\Framework\Model\AbstractModel->save() #20 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-sales/Controller/Adminhtml/Order/Invoice/Save.php(192): Magento\Framework\DB\Transaction->save() #21 /home/my_username/domains/my_domain.com.nl/public_html/var/generation/Magento/Sales/Controller/Adminhtml/Order/Invoice/Save/Interceptor.php(24): Magento\Sales\Controller\Adminhtml\Order\Invoice\Save->execute() #22 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/Action/Action.php(102): Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor->execute() #23 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-backend/App/AbstractAction.php(226): Magento\Framework\App\Action\Action->dispatch(Object(Magento\Framework\App\Request\Http)) #24 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(74): Magento\Backend\App\AbstractAction->dispatch(Object(Magento\Framework\App\Request\Http)) #25 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(70): Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor->___callParent('dispatch', Array) #26 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(63): Magento\Framework\Interception\Chain\Chain->invokeNext('Magento\Sales\C...', 'dispatch', Object(Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor), Array, 'adminAuthentica...') #27 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-backend/App/Action/Plugin/Authentication.php(143): Magento\Framework\Interception\Chain\Chain->Magento\Framework\Interception\Chain\{closure}(Object(Magento\Framework\App\Request\Http)) #28 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(67): Magento\Backend\App\Action\Plugin\Authentication->aroundDispatch(Object(Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor), Object(Closure), Object(Magento\Framework\App\Request\Http)) #29 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Framework\Interception\Chain\Chain->invokeNext('Magento\Sales\C...', 'dispatch', Object(Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor), Array, 'adminMassaction...') #30 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-backend/App/Action/Plugin/MassactionKey.php(33): Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http)) #31 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(142): Magento\Backend\App\Action\Plugin\MassactionKey->aroundDispatch(Object(Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor), Object(Closure), Object(Magento\Framework\App\Request\Http)) #32 /home/my_username/domains/my_domain.com.nl/public_html/var/generation/Magento/Sales/Controller/Adminhtml/Order/Invoice/Save/Interceptor.php(39): Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor->___callPlugins('dispatch', Array, Array) #33 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/FrontController.php(55): Magento\Sales\Controller\Adminhtml\Order\Invoice\Save\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http)) #34 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(74): Magento\Framework\App\FrontController->dispatch(Object(Magento\Framework\App\Request\Http)) #35 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(70): Magento\Framework\App\FrontController\Interceptor->___callParent('dispatch', Array) #36 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Framework\Interception\Chain\Chain->invokeNext('Magento\Framewo...', 'dispatch', Object(Magento\Framework\App\FrontController\Interceptor), Array, 'install') #37 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Module/Plugin/DbStatusValidator.php(69): Magento\Framework\App\FrontController\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http)) #38 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(142): Magento\Framework\Module\Plugin\DbStatusValidator->aroundDispatch(Object(Magento\Framework\App\FrontController\Interceptor), Object(Closure), Object(Magento\Framework\App\Request\Http)) #39 /home/my_username/domains/my_domain.com.nl/public_html/var/generation/Magento/Framework/App/FrontController/Interceptor.php(26): Magento\Framework\App\FrontController\Interceptor->___callPlugins('dispatch', Array, Array) #40 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/Http.php(135): Magento\Framework\App\FrontController\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http)) #41 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/Bootstrap.php(258): Magento\Framework\App\Http->launch() #42 /home/my_username/domains/my_domain.com.nl/public_html/index.php(40): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http)) #43 {main} [] [] [2018-02-15 13:46:44] main.CRITICAL: Magento\Framework\View\Asset\File\NotFoundException: Unable to get content for 'frontend/Magento/luma/nl_NL/Cis_Comment/js/custom.min.js' in /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Asset/File.php:165 Stack trace: #0 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Asset/MergeStrategy/Direct.php(73): Magento\Framework\View\Asset\File->getContent() #1 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Asset/MergeStrategy/Direct.php(52): Magento\Framework\View\Asset\MergeStrategy\Direct->composeMergedContent(Array, Object(Magento\Framework\View\Asset\File)) #2 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Asset/MergeStrategy/FileExists.php(44): Magento\Framework\View\Asset\MergeStrategy\Direct->merge(Array, Object(Magento\Framework\View\Asset\File)) #3 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Asset/Merged.php(97): Magento\Framework\View\Asset\MergeStrategy\FileExists->merge(Array, Object(Magento\Framework\View\Asset\File)) #4 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Asset/Merged.php(157): Magento\Framework\View\Asset\Merged->initialize() #5 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Page/Config/Renderer.php(358): Magento\Framework\View\Asset\Merged->rewind() #6 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Page/Config/Renderer.php(251): Magento\Framework\View\Page\Config\Renderer->renderAssetHtml('<script  type="...', Object(Magento\Framework\View\Asset\Merged)) #7 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Page/Config/Renderer.php(226): Magento\Framework\View\Page\Config\Renderer->renderAssetGroup(Object(Magento\Framework\View\Asset\PropertyGroup)) #8 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Page/Config/Renderer.php(99): Magento\Framework\View\Page\Config\Renderer->renderAssets(Array) #9 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Result/Page.php(235): Magento\Framework\View\Page\Config\Renderer->renderHeadContent() #10 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/View/Result/Layout.php(164): Magento\Framework\View\Result\Page->render(Object(Magento\Framework\App\Response\Http\Interceptor)) #11 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(74): Magento\Framework\View\Result\Layout->renderResult(Object(Magento\Framework\App\Response\Http\Interceptor)) #12 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(70): Magento\Framework\View\Result\Page\Interceptor->___callParent('renderResult', Array) #13 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(63): Magento\Framework\Interception\Chain\Chain->invokeNext('Magento\Framewo...', 'renderResult', Object(Magento\Framework\View\Result\Page\Interceptor), Array, 'result-varnish-...') #14 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-page-cache/Model/Controller/Result/VarnishPlugin.php(74): Magento\Framework\Interception\Chain\Chain->Magento\Framework\Interception\Chain\{closure}(Object(Magento\Framework\App\Response\Http\Interceptor)) #15 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Chain/Chain.php(67): Magento\PageCache\Model\Controller\Result\VarnishPlugin->aroundRenderResult(Object(Magento\Framework\View\Result\Page\Interceptor), Object(Closure), Object(Magento\Framework\App\Response\Http\Interceptor)) #16 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Framework\Interception\Chain\Chain->invokeNext('Magento\Framewo...', 'renderResult', Object(Magento\Framework\View\Result\Page\Interceptor), Array, 'result-builtin-...') #17 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/module-page-cache/Model/Controller/Result/BuiltinPlugin.php(67): Magento\Framework\View\Result\Page\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Response\Http\Interceptor)) #18 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/Interception/Interceptor.php(142): Magento\PageCache\Model\Controller\Result\BuiltinPlugin->aroundRenderResult(Object(Magento\Framework\View\Result\Page\Interceptor), Object(Closure), Object(Magento\Framework\App\Response\Http\Interceptor)) #19 /home/my_username/domains/my_domain.com.nl/public_html/var/generation/Magento/Framework/View/Result/Page/Interceptor.php(130): Magento\Framework\View\Result\Page\Interceptor->___callPlugins('renderResult', Array, Array) #20 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/Http.php(139): Magento\Framework\View\Result\Page\Interceptor->renderResult(Object(Magento\Framework\App\Response\Http\Interceptor)) #21 /home/my_username/domains/my_domain.com.nl/public_html/vendor/magento/framework/App/Bootstrap.php(258): Magento\Framework\App\Http->launch() #22 /home/my_username/domains/my_domain.com/public_html/index.php(40): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http)) #23 {main} [] [] 

Anybody has a clue of what is going wrong here?