I found this in my logs:

Script: [...] /board/scripts/domovethread.inc.php - Line: 316
UPDATE ubbt_ANNOUNCEMENTS
SET FORUM_ID = 30
WHERE TOPIC_ID = 11593
- Duplicate entry '11593-30' for key 1

I seems the mod work on a locked sticky thread and try to move them to another forum.

Will need to look at that. If it has created an issue with that particular topic I did add some code in 7.5.4 that does some sanity checking on the announcements table when you use the Rebuild Topics option in the control panel.

I fixed it that way (hope to remember correct) that I unstick the thread, post a dummy reply to it. The dummy becomes the new first post but be sorted in dates. After this I can merge and move all as it should.