Go to your UCP – manage groups – manage users

Try to add a known user and hit submit.

Error:

Fatal error: Call to undefined function cache_moderators() in /var/www/web16/html/area51/includes/functions_user.php on line 2227

The user itself was added to the group.

Only the function cache_moderators is not defined.
( functions_admin.php )

From ACP it works without error.