"Fatal exception of type MWException" as answer for "Special:ManageMessageGroups" page
Fragment of a discussion from Support/LiquidThreads
Dieudo, simple update problems for stats pages are solved as usual in MediaWiki, by appending action=purge (and then refreshing a few times without action=purge in the URL, to fill the stats); more complex problems like those you describe are similar to an issue related to local browser cache (please hard refresh the page) which we had on our wikis too and was IIRC solved in more recent code/configuration, so please paste relevant configuration from your LocalSettings.php if nothing solves it.