FYI new migrated forum is up and running great :)
(to clarify, this was not the same parent_node_id undefined problem as mentioned before.. that part was fine in our install)
Yea got it too vB4.2.5 -> 2.1.10 :rolleyes:
Traced it back to src/XF/Behavior/TreeStructured.php preSave() function and simply commented the error line out
$newParent = $entity->em()->find($entity->structure()->shortName, $parentId)...