Well, I am not sure what the problem is, but I think both you and slim_cutty have your forums inside a "forums" directory, instead of within the root directory of your URL.
Try this and let me know how it works:
Edit your Warn.php file and find this:
PHP Code:
$url="/$admincpdir/user.php?do=edit&userid={$_POST['id']}";
Try changing that to:
PHP Code:
$url="$admincpdir/user.php?do=edit&userid={$_POST['id']}";
Let me know how that works. If it does not solve your problem, please go again in the User page and scroll all the way to the bottom, where there is the Warnings Information box. You should have a button there saying "Warn (non-post related). Hover your mouse over that button and see what it says at the status line of your browser. Tell me that.
Rgds