We have a shard cluster with 3 QR nodes, one PSS config setup and 1 shard setup(1 primary and 4 secondary servers) with 5.0.7 version on all the nodes.
We can see the 'logicalSessionRecordCache.activeSessionsCount' is increasing frequently and not getting cleared on all the QR and the data nodes. While checking from the serverStatus, it shows the number of sessions refreshed and the number of sessions ended during the last refresh was 0. 
 

 
So we checked the 'config.system.sessions' collections and it showed an error "No chunks were found for the collection config.system.sessions" and we are getting the same error message in the log also.
 

 
We faced the connection increase in shard setup and the LogicalSessionRecordCache.activeSessions increased to the max limit and mongo became unresponsive. (limit 1000000).
 
 
 
To check the session related details we need to access the  'config.system.sessions' collection. Which is not accessible and giving the error as mentioned. 
Need the solution to access the 'config.system.sessions' collection and need the solution for why 'logicalSessionRecordCache.activeSessionsCount' is increasing frequently and not getting cleared (flushed).