Like the programmers on the Jersey Shore, I like to follow a GTL approach to debugging cognos issues.
Google. Cognos is an off the shelf product and has standard error messages. Anything you might see there's a guy with a blog explaining how to fix it. *cough*. Google that error code, message text or other identifying issue.
Ticket. Open a ticket with Cognos if you have a support contract. They are usually not the quickest in coming up with a solution so I like to get this started early. Also check that there hasn't been a fix pack released that addresses your issue. Many things, like difficult to debug server stability issues, are constantly being updated by fix packs.
Logging. Enable more detailed logging on the server side if you need more context for the error message. And repeat!
(Yes I made this post because I thought of an acronym)