Why do I get this error after editing a post and then trying to re-save it?

Why do I get this error after editing a post and then trying to re-save it?

The item Corona SDK is only a container for forums. Please select one of the forums below it.

Then I go to the pop-up list to select a forum and only 2 item are on it: “General” and “Corona SDK”. [import]uid: 295 topic_id: 1526 reply_id: 301526[/import]

Hm. Added case # 447 for our web experts to look at. [import]uid: 54 topic_id: 1526 reply_id: 4300[/import]