OK, so you get error stack-traces in a *debug* log, and in the error log you get warning notices? *facepalm*
I have a stack-trace which may be relevant.
[error] [YNT6JB0y@yYaYYse7zE8PgAAAIc] /neulatein/index.php?title=Spezial:XML_anzeigen&categories%5BLBI-Authors%5D=on&namespaces%5B0%5D=on ErrorException from line 139 of /sites/
wiki.uibk.ac.at/app/bluespice-20191215/local/extensions/DataTransfer/includes/DT_PageStructure.php: PHP Notice: Undefined index: 0505261
#0 $IP/local/extensions/DataTransfer/includes/DT_PageStructure.php(139): MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 $IP/local/extensions/DataTransfer/includes/DT_PageStructure.php(26): DTPageStructure->parsePageContents(string)
#2 $IP/local/extensions/DataTransfer/includes/specials/DT_ViewXML.php(138): DTPageStructure::newFromTitle(Title)
#3 $IP/local/extensions/DataTransfer/includes/specials/DT_ViewXML.php(230): DTViewXML->getXMLForPage(Title, NULL)
#4 $IP/local/extensions/DataTransfer/includes/specials/DT_ViewXML.php(22): DTViewXML->doSpecialViewXML(NULL)
#5 $IP/includes/specialpage/SpecialPage.php(565): DTViewXML->execute(NULL)
#6 $IP/includes/specialpage/SpecialPageFactory.php(568): SpecialPage->run(NULL)
#7 $IP/includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext)
#8 $IPincludes/MediaWiki.php(861): MediaWiki->performRequest()
#9 $IP/includes/MediaWiki.php(524): MediaWiki->main()
#10 $IP/index.php(42): MediaWiki->run()
#11 {main}
That index is very suspicious. Perhaps the page content is so big that it triggers some kind of numeric overflow?