Using Business Objects Enterprise XI 3.1 Service Pack 5.
Getting the error message: The document cannot be created or opened. The error code is: File EXISTS BUT IS INVALID. when opening web intelligence reports.
The output cache is timing out due to a network connection which has been queued. If any network traffic is there, then the pending requests gets delayed and accordingly the session gets timed out and throw the error.
Disable caching for the Web Intelligence Processing Server
Make sure that the network share is close to the Processing Server and that the network can handle heavy traffic without latency.