MediaWiki will not save preferences
MediaWiki does not save preferences
I've seen this a few times when I've been tinkering with Apache domain name settings. In my case it's been caused by the default server name being
http://www.is-beer-a-vegetable.com rather than
http://is-beer-a-vegetable.com or vica versa. www or non-www.
This behaviour may be modified by editing Apache settings or possibly by your server provider's admin panel.
It is also worth checking the setting of