Create guest account and comu forum error

All of a sudden User->CreateGuest did not work anymore.
It returned "HTTP/1.1 500 Internal Server Error".

I had a look through the code and found the problem in users.php

// Notify the addons
AddonModule::NotifyUserCreate($user);

Since I recently purchased and installed Combu Forum this was calling \addons\forum\addon.php.
I think this code should check if the user is a guest user and if so not execute

function Forum_OnUserCreate($user) {
    // Subscribe new accounts to default group
	
    $userGroups = Forum_UserGroup::Load();
    foreach ($userGroups as $group) {
        if ($group->DefaultGroup) {
            $rec = new Forum_UserGroup_Account();
            $rec->IdAccount = $user->Id;
            $rec->IdUserGroup = $group->Id;
            $rec->Save();
            break;
        }
    }
}