I just found this thread about Mixed sessions. We are experiencing the same problem with RT 3.8.7, sitting behind an Apache based proxy + load balancer. The backend is a MySQL cluster but that should not have an impact on the user sessions.
We have two instances of RT connected to the same backend. In other words, the front ends connect (load balance) to two RTs. These RTs connect to the same Cluster. Could the apache load balancer be the issue? Suggestions? Thierry