Open a thread and click on the checkbox on the first post.
Permanently delete it and the browser refreshes back to the thread URL and an error message is displayed.
This only happens when deleting within the thread, not from the forum view.
So if I understand you right, you're NOT deleting a thread from within the thread, but you delete one or more post(s) with the result of the whole thread being deleted. This might be the difference and this is where the error occurs!
When you use the thread tools to delete the thread from within the thread, everything is ok.