When posting a rating with debugging turned to ALL I received the following PHP notice

 Notice: Undefined property: stdClass::$cmidnumber in /home/arborrow/Moodle/code/19stable/mod/forum/lib.php on line 1168

This is caused because the forum_grade_item_update function in /mod/forum/lib.php expects the $forum variable to have a field $cmidnumber ( * @param object $forum object with extra cmidnumber). I traced this back to /mod/forum/rate.php and found that the extra cmidnumber field was not being added on prior to passing the variable to the forum_grade_item_update function.

I'll attach one possible patch that I used to get around this.