This is correct, [ and ] are not allowed for page names, so you can't create an account containing them in its name. The plugin that allows you to log in with your forum's account should be updated to handle that.

Marking this INVALID. Error with custom site extension (nothing's listed on http://wiki.alliedmods.net/Special:Version, so it's not one from our SVN ones).

To the reporter: I suggest talking to your site administrators and tell them that their plugin needs to check that a username is a valid Title object (pass the usernames to Title::newFromText()) before attempting a login. As comment #1 notes, [] is invalid in titles (and thus usernames) in MediaWiki.