MediaWiki internal error.

Original exception: [b7e994a8bc361af9c0b77be7] /index.php?title=UserWiki:Poolparty&oldid=19639 Wikimedia\Assert\PostconditionException from line 202 of /var/www/primepedia.de/vendor/wikimedia/assert/src/Assert.php: Postcondition failed: makeTitleSafe() should always return a Title for the text returned by getRootText().
Backtrace:
#0 /var/www/primepedia.de/includes/Title.php(1986): Wikimedia\Assert\Assert::postcondition()
#1 /var/www/primepedia.de/includes/skins/Skin.php(480): Title->getRootTitle()
#2 /var/www/primepedia.de/includes/OutputPage.php(3128): Skin->getPageClasses()
#3 /var/www/primepedia.de/includes/skins/SkinTemplate.php(405): OutputPage->headElement()
#4 /var/www/primepedia.de/includes/skins/SkinTemplate.php(127): SkinTemplate->prepareQuickTemplate()
#5 /var/www/primepedia.de/includes/skins/SkinTemplate.php(144): SkinTemplate->generateHTML()
#6 /var/www/primepedia.de/includes/OutputPage.php(2622): SkinTemplate->outputPage()
#7 /var/www/primepedia.de/includes/MediaWiki.php(952): OutputPage->output()
#8 /var/www/primepedia.de/includes/MediaWiki.php(965): MediaWiki->{closure}()
#9 /var/www/primepedia.de/includes/MediaWiki.php(548): MediaWiki->main()
#10 /var/www/primepedia.de/index.php(53): MediaWiki->run()
#11 /var/www/primepedia.de/index.php(46): wfIndexMain()
#12 {main}

Exception caught inside exception handler: [b7e994a8bc361af9c0b77be7] /index.php?title=UserWiki:Poolparty&oldid=19639 Wikimedia\Assert\PostconditionException from line 202 of /var/www/primepedia.de/vendor/wikimedia/assert/src/Assert.php: Postcondition failed: makeTitleSafe() should always return a Title for the text returned by getRootText().
Backtrace:
#0 /var/www/primepedia.de/includes/Title.php(1986): Wikimedia\Assert\Assert::postcondition()
#1 /var/www/primepedia.de/includes/skins/Skin.php(480): Title->getRootTitle()
#2 /var/www/primepedia.de/includes/OutputPage.php(3128): Skin->getPageClasses()
#3 /var/www/primepedia.de/includes/skins/SkinTemplate.php(405): OutputPage->headElement()
#4 /var/www/primepedia.de/includes/skins/SkinTemplate.php(127): SkinTemplate->prepareQuickTemplate()
#5 /var/www/primepedia.de/includes/skins/SkinTemplate.php(144): SkinTemplate->generateHTML()
#6 /var/www/primepedia.de/includes/OutputPage.php(2622): SkinTemplate->outputPage()
#7 /var/www/primepedia.de/includes/exception/MWExceptionRenderer.php(153): OutputPage->output()
#8 /var/www/primepedia.de/includes/exception/MWExceptionRenderer.php(65): MWExceptionRenderer::reportHTML()
#9 /var/www/primepedia.de/includes/exception/MWExceptionHandler.php(106): MWExceptionRenderer::output()
#10 /var/www/primepedia.de/includes/exception/MWExceptionHandler.php(185): MWExceptionHandler::report()
#11 /var/www/primepedia.de/includes/MediaWiki.php(581): MWExceptionHandler::handleException()
#12 /var/www/primepedia.de/index.php(53): MediaWiki->run()
#13 /var/www/primepedia.de/index.php(46): wfIndexMain()
#14 {main}