This error happens when somehow your app is trying to load a class, and the classloader being asked to do it is a stale one left around from an earlier deployment of the webapp. Perhaps you redeployed the app but something is holding a ref to a class loaded from the previous deployment.
A quick solution is probably to restart the server.
Sorry I can't provide more details than this, e.g. how to find exactly what the cause is. Suggest you post on the tomcat forum here at JBoss, or on the tomcat-user mailing list. Be sure to mention what release you are running, as that *could be* very relevant.