SSRS 2012: "The report execution has expired or cannot be found. (rsExecutionNotFound)"
This might help someone.
In my case, The report url had trailing spaces (a silly mistake) which caused this.
I know this is an old question but I had this problem recently and it turned out to be a bad session cookie. The cookies session-id matched the guid in the error message and once I deleted the cookie all worked fine after that. The report at one point had been configured to cache a temporary copy
but that had since been turned off (however, the problem existed before that had been turned off so it may not be relevant).
Hopefully this answer will help someone else save the hour I spent figuring it out in my environment :)
I've added &rs:Command=ClearSession to the end of my url and works fine with me.