WARNING Context manager doesn't exist

Hi All,
I am using tomcat 5.0.27 as a three-node cluster. I am having a system where I use a login box and the UserName and PassWord I keep in a session object.
The Problem I am having is that after some time I get the message

WARNING: Context manager doesn't exist

And I cannot use the session objects. My session object becomes null.
Even if I try to use a new browser window and try to re-login that also gives the same error.

If my existing session expires then I can understand as my session might get expired due the expire duration set in the web.xml file.

But the problem lies even a new session cannot be created, as all sessions become null.

When I re-start the cluster, for a while it is ok, then the session gets destroyed and it becomes null.

How ever If I shut down the cluster and run the system as a single Node tomcat instance, then everything is ok. So is what I am experiencing a cluster problem.

As this sounds like a Tomcat-specific problem, 'm going to move this to our Apache/Tomcat forum.