Role base security is throwing an exception when the session lock timer fires. See attached.

Hi Larry.

I tried many different scenarios here to try and reproduce your problem and haven't been able to.
At this point, the framework is adding your open forms and their WindowStates to a dictionary, and apparently there is already a key of the same type added there.
Are you calling SessionLock.LockSession() based on a timer that could be triggering more than once, while on the second time the application would be already on a Locked state?

Cheers.