Judging from the error:
You don't have permission to access /forum/admin/changeuser.php on this server.
I'm wondering what the permissions are on that file; are you able to preform this same activity on other members? Are there any items showing up in your web server error log when you're getting this forbidden message?
The forbidden message is generated by your web server and isn't generated by the UBB; we simply need to figure out WHY it's generating the error message.