I don’t have any error when accessing https://nandeck.myxwiki.org/xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin§ion=userdirectory, so I assume it has something to do with the fact that I have global admin right (which is also the case when you try on a local demo package like you did). You still reproduce the problem on myxwiki.org, right?
Could you open your browser’s Web Developer Tools, reload the page and see if you have a request with an error in the Network tab? If you click on it, you might have a more detailed error in the “Response” tab on the right.