The exception you get means that their login session expires while they are using the backend. This shouldn't happen if you are using the default Joomla! administrator templates ("Isis") since it automatically "pings" the server every few seconds to keep the session alive. So, if you're not using the default administrator template or nothing else seems to work try increasing the session time in Global Configuration to, say, 30 minutes.
The other thing that can get in the way is session cleanup. For example, the one you can configure in the options of the System - Admin Tools plugin. Try setting a longer time between session cleanups. It doesn't make sense to do it more often than once per hour. Ideally you should run it just once per day.
Nicholas K. Dionysopoulos
Lead Developer and Director
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