Cause
An XDQP message referenced an XDQP session that does not exist. This can happen if an XDQP session timed out (for example, if the network is really busy) or if an XDQP session was terminated (for example, if a node restarted).
Response
If the message shows as a debug message in the ErrorLog.txt file, and if there are a small number of these messages, then it indicates that there was a transient problem that corrected itself. If you see many messages for a large period of time, it might be an indication of a system that is resource bound, and you should find the cause of the resource contention and correct it.
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