Tim,
The "workaround" that's been suggested in the past was to have them log out and log back in, and everything should work. Something is forcing the session key for the users browser session to fail, and a logout/login will reset the key and clear the problem.
It's never been explained whether it's a false time-out, a glitch that somehow invalidates the key, or exactly why the key is becoming invalid, but in our shop, every time it happens we just log out, log back in and everything's good to go...It can be a little frustrating if you're making large edits to description fields / notes fields, but generally, it's a pretty quick fix.
Steve