Several things going on here. #1 and #3 are related. Check your /includes/config.inc.php file and look for the SESSION_PATH variable. That appears to be wrong and it's creating the registration issue as well as the error.
The one resulting in the SQL error appears that the user has access to view user profiles but doesn't have access to view any forums. Is this a guest user, or one that's registered? If registered, then check if they belong to any groups. If it's unregistered and the user cannot view any forums (and that's the way you want it), then they probably shouldn't be able to view user profiles either.
