We are running tomcat application server that handle over 100 concurrent sessions. In the last 2 months the most active users noticed that sometimes they are getting kicked
Eventhough I do not know the cause of the problem, one possible fix (which I had done at my previous project) would be to run the application on a tomcat cluster and have session failover. Sessions can be by default sticky and when one node goes down, the healthy nodes pick up the sessions and all this is transparent to the end user.