When I remove the folder from my projects, it disappears but it is temporary. After reloading the page, it's back.
I didn't find it in search
... and I don't understand...
Just tested I see the same behaviour
When I delete my old folders, they disappear. However, after reloading the page, they reappear. Here's a video:
Using Chrome
No issue here (with Firefox).
Ah, there is a trick to this.
Put a project in the folder you want to delete. Then delete the folder (send to trash). The folder will then be gone, but the project will be in the Trash.
Oh, it was because they were empty...
Interestingly, it's the just opposite of how it works with a project. There, folders can only be deleted if they are empty or have been emptied beforehand.
(At least if you don't use a specific file extension that allows this even for non-empty folders.)
Empty folders should be deleted. This was a bug in the past, but I thought I fixed it. I'll file a bug.