In a thread with a lot of pages, i.e. over 70. I click the button to go to the first unread post and I get:

Server Error
500 - Internal server error.
There is a problem with the resource you are looking for, and it cannot be displayed.

This is consistent in that it always happens. I have not determined exactly what the limit is before this happens but it isn't a lot of pages.

I am RCU1 but that doesn't seem to matter. It happens regardless of the server I am on.