[JulianD]

There's a bug (or something working wrong with my live and dev sites):

You're inserting the requested nick to the database in an escaped form, but once the admin approves the nick, it's given escaped to the datamanager. For example:

<julian> gets inserted as &lt;julian&gt; to the database. That's ok.

But once the admin approves the change, you're passing the escaped string without unescaping it. So vbulletin thinks that the username to change is &lt;julian&gt; and will produce an error.