Cannot lock session scope.
CFLock cannot be used to lock the application or session shared scopes without these scopes being established through the use of the CFApplication tag. If you want to use the session scope you need to enable session management. Application and/or Session variables must also be enabled in the ColdFusion Administrator.
Most likely you either do not have sessionmanagement enabled in your Application.cfm or Application.cfc or you are trying to use a session lock within your onApplicationStart() method of Application.cfc.
This is a reminder and a gotcha that session scope does not exist until onApplicationStart() finishes executing. The interesting thing also, is that in Railo 3.1 session does exist!
Anyways, beware the lock of session or acessing session from onApplicationStart().