Default user session timeout is 240 minute.This issue doesn't seem to be a user timeout issue.
It is more of workbook interaction with the server.
What are you trying to do within the workbook.
That's a new one. What is the data source? What version of Server and what version of Desktop was used to publish?
I have faced the same issue long back when i was using 9.2 due to network issue it happened, but i don't know what might be the the reason in your situation.
According to my knowledge it might be the improper interaction of workbook with the server.
few questions wee need to know before digging into it is..
1) what type of workbook it is (Live or Extract)?
2) Have your Administrator changed your permissions?
I hope this helps a little bit.