Issue created here:
https://jira.xwiki.org/browse/ADMINTOOL-59
Actually XWiki 11.5 should be ok as a stable version; it is the Admin Tools which needs to be updated. This extension uses things that are not part of any API and might break between XWiki versions, and thus it needs updates here and then. The configuration check is unlikely to show you anything interesting, as the default values are nowadays usually ok, unlike somewhat five to ten years ago, where one usually had the “Oh, there I need to enable UTF-8, too”-experience quite often
If you really want an XWiki version that works better with the Admin-Tools, you might try the “Long Term Support” version, where the current version is 10.11.9:
https://www.xwiki.org/xwiki/bin/view/Download/DownloadVersion/?projectVersion=10.11.9