Do you have a load balancer using cookie affinity in front of a cluster of Lightstreamer servers?
If so, error 21 could indicate that when a client returns from the background and tries to recover the session, it reaches the wrong server, i.e., a server other than the one that created the original session.
To prevent this error, you should set the CookieHandlingRequired parameter to true and ensure that the load balancer is correctly configured to set and read affinity cookies upon the client's requests.
