Last modified: 2008-02-28 00:05:12 UTC
The way Special:Userrights is written causes it to stop reading username once it reaches a @ symbol, and then check for the -interwiki right. While this would work on a new installation, it lacks backwards compatibility for wikis that have users with an @ symbol in their username, before this was disallowed. The software thinks you are changing a user at another wiki, not a local user with @ in their name. While a user with the -interwiki right might be able to do username@whatever@wikiname [not tested], this is still an issue.
The bug is that the at-sign hasn't yet been removed from existing usernames. It's supposed to be invalid. See bug 12581 (which would also resolve this, so I'll mark this as duplicate).
*** This bug has been marked as a duplicate of bug 12581 ***
This can be considered fixed by the fix for bug 13135.
*** This bug has been marked as a duplicate of bug 13135 ***