Had the same issues as presented above.
Our quick fix was also:
Admin>System Settings>Advanced>Validate user IP address: TURN OFF
However, in the environment this CRM sits in it didn’t allow for us to keep the Validate user IP address OFF as a permanent fix.
After looking at logs the cause was a new install of Chrome where the user logged into their profile. When chrome brought over the existing cookies it would cause the system to log them out every 15 mins or so.
The FIX: Clear your cookies and log in. Test, if the logouts stop. Then turn back on Validate user IP address, as per the work @RickfiregamesPT noted. We were also able to verify his work. Cheers! Hope this helps someone.