Unable to move posts due to 500 error

[Canapin (Coin-coin le Canapin)]

I found a similar issue that was fixed 5 years ago:

Moving a post fails in unknown circumstances

I’ve noticed the same problem with imported posts. They do not always conform to requirements like min_post_length. Moving posts triggered those validations and silently prevented the posts from being moved to the target topic. It should be fixed. https://github.com/discourse/discourse/commit/0ecdf900230eee21beaf54b80428bd18768127ae

Is your forum imported?