The root cause is that the StandardServer.CloseSession[1] method is passing the Session Id to SessionManager.CloseSession[2], but it is expecting the Authentication Token (both are of type NodeId). The SessionManager.CloseSession method attempts to look up the session in the dictionary of sessions but it is not found because the wrong key is being used (the SessionManager.CreateSession[3] method initially adds the session to the dictionary using the Authentication Token as the key).

The result is that the session is never actually closed. If a session with the same name is created, it ends up reusing the same Session Id. If the new session establishes a subscription, problems occur. Specifically, after the original session expires (by default, after 18 minutes of inactivity, as detected by SessionManager.MonitorSessions[4]), subscriptions for the new session stop reporting updates.

[1] public void CloseSession(OperationContext context, NodeId sessionId, bool deleteSubscriptions)
C:\Program Files\OPC Foundation\UA SDK 1.00\Source\Common\Server\Server\StandardServer.IServerInternal.cs

[2] public virtual void CloseSession(NodeId authenticationToken)
C:\Program Files\OPC Foundation\UA SDK 1.00\Source\Common\Server\Session\SessionManager.cs

[3] public virtual Session CreateSession(…)
C:\Program Files\OPC Foundation\UA SDK 1.00\Source\Common\Server\Session\SessionManager.cs
[Not to be confused with 'protected virtual Session CreateSession(…)']

[4] private void MonitorSessions(object data)
C:\Program Files\OPC Foundation\UA SDK 1.00\Source\Common\Server\Session\SessionManager.cs