Issue in one concise sentence
Steps to reproduce
Server is 6.6.1. Desktop client is 5.0.4. I have reports of this happening with the web browser as well.
The little blue indicator over “Unreads” on the Threads page should only be there if you have unread threads.
The little blue indicator over “Unreads” appears, but there are no unread messages and the message in the right pane shows a negative number of unread threads.