rsExecutionNotFound when directly accessing an SSRS report
What I have seen in a case like you ask about is you hit a report, and copy the url out and then try to hit it later. Reports have session state, especially if you change paramaters,etc. The url will look like a report url with some funky querystring data added to the end. You can't share that because SSRS can't take your "session state" and then find it later.