Recently, I struggled to move a post to a new topic. The modal only showed:
There was an error moving posts.
Well, this time it was not that difficult; I hadn’t chosen a category. But, fun fact, it didn’t work when I added a category either. So I checked the browser console:
That didn’t help me either, but finally, in the network tab, I found:

This would have been really helpful to know without using a device where I have access to such tools. Is it possible to show the actual error message in the interface?
Related off-topic question
That would have really helped me to understand why I am sometimes able to split into a new topic with the title “Testing” and sometimes not.
Then I would have known it’s the character count and it would have been one step closer to think that there might be a difference if the first selected post was created by an admin. Is it expected that an admin can bypass the min topic title length while splitting a post if the post is written by an admin but not if it was created by a user?

