The requested thread could not be found.
The thread had not been deleted and I had permissions to see that thread.
When I checked the link from the email the number the URL had the correct thread subject title but the wrong number.
e.g. Link from email = http://domain.com/threads/thread-title.13996/unread
Actual thread link = http://domain.com/threads/thread-title.13995/
Can anyone help please?