Moved to .NET SDK forum
Could be resources in IIS. Memory fragmentation also.
Or too many jobs open at one time. Make sure you are closing and disposing the report objects after each job is done and calling GC.Collect to force Windows to free up the memory marked to be freed.
Without more logs can't say exactly what the issue was.
Typically though when that error is thrown it's due to no more jobs requests are available or there is a communication error between IIS and the BOE Server if you are using Managed RAS. Or could even be the report ID no longer exists.
Don