See support desk posting where @Semako96 writes:

Sometimes when I try to upload multiple files with Extension:Multiupload, I get the following error:

[0d8ea9145562260a12e68c68] /wiki/Spezial:MultiUpload Wikimedia\Assert\ParameterTypeException from line 89 of .../vendor/wikimedia/assert/src/Assert.php: Bad value for parameter $dbkey: must be a string

Backtrace:

#0 .../includes/title/TitleValue.php(80): Wikimedia\Assert\Assert::parameterType(string, NULL, string)
#1 .../includes/specialpage/SpecialPage.php(100): TitleValue->__construct(integer, NULL, string)
#2 .../includes/specialpage/SpecialPage.php(84): SpecialPage::getTitleValueFor(NULL, boolean, string)
#3 .../includes/specialpage/SpecialPage.php(630): SpecialPage::getTitleFor(NULL, boolean)
#4 .../includes/specials/SpecialUpload.php(255): SpecialPage->getPageTitle()
#5 .../extensions/MultiUpload/SpecialMultiUpload.php(476): SpecialUpload->getUploadForm(string, string, boolean)
#6 .../includes/specials/SpecialUpload.php(524): UploadRow->showUploadWarning(array)
#7 .../extensions/MultiUpload/SpecialMultiUpload.php(339): SpecialUpload->processUpload()
#8 .../extensions/MultiUpload/SpecialMultiUpload.php(98): UploadRow->handleRequestData()
#9 .../extensions/MultiUpload/SpecialMultiUpload.php(78): SpecialMultiUpload->handleRequestData()
#10 .../includes/specialpage/SpecialPage.php(522): SpecialMultiUpload->execute(NULL)
#11 .../includes/specialpage/SpecialPageFactory.php(568): SpecialPage->run(NULL)
#12 .../includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext)
#13 .../includes/MediaWiki.php(861): MediaWiki->performRequest()
#14 .../includes/MediaWiki.php(524): MediaWiki->main()
#15 .../index.php(42): MediaWiki->run()
#16 {main}

Wiki runs with MediaWiki 1.31 on php 7.2 with MySQL 5.5 InnoDB.

What's the problem there, how to fix it?