I have enabled the “Introducing permanently delete post functionality”. I can delete most of the posts. However, there are some posts where the following message appears:
An error has occurred: This topic has recovered posts. Please delete them permanently before deleting the topic permanently.
However, all other posts in the thread are deleted and there are no restored posts either.
Any help?
I tried Safe-Mode, analyzed logs. Nothing. Any infos I can provide to solve this?
Can you install data explorer and execute this query?
SELECT id, raw
FROM posts
WHERE topic_id = XXX
Replace XXX with your topic’s ID, and see what it returns. It may give clues 
Thank you, that helped! Apparently there are still references to PMs or some of the threads have been converted to PMs.
Good to know! I wouldn’t have guessed that 
Did this solve it for you in the end?
No. Still one topic left I can’t delete.