We encountered this error in our 3.1 environment when working in webi rich client, not a lot, but enough to make us scratch our heads.
We never did establish the root cause, but high likely a timeout issue as we changed a setting in the webi process server (Idle Document Timeout and set it to 1800 seconds).
This is not nesseccarily a fix, but merely a workaround as it may work for some and not for others.