SharePoint “User not found” error with Reporting Services in integrated mode

I encountered an error the other day while trying to get a SharePoint 2007 (WSS 3.0) site running with SQL Server Reporting Services 2005 in integrated mode.  While trying to access any report .RDL files inside a document library, I got the “User not found” error from SharePoint.  After looking in the logs, I could not find anything.  What I did note, was that the development site worked, but the production site did not.

One difference between them – the names of the Application Pool that SharePoint was running under.  In development it was just “spsite” but in production it was “www.spsite.com”.

I renamed the application pool and *poof*, the site began to work.  I don’t understand the mechanism, but just thought I’d post in case someone else out there encounters it.

UPDATE: I’ve blogged about another reason why you might get the “User not found” error and possible solutions.