Thread:Project:Support desk/User preferences won't save (no error)/reply (16)

Your wiki has Short URLs wrongly configured, or there's something wrong with server alias or rewrite rules.

When saving Special:Preferences it does a HTTP POST to /index.php/Special:Preferences, but the server is responding with a HTTP 302 Redirect to /Special:Preferences, which causes the browser to do a GET request instead of a POST one and the POST data is lost on the redirect.