Check that no one is accessing sage 100 over a wireless connection (not supported by sage and will only cause headaches).

Check that network cards are not going into power save mode, in other words make sure nothing will cause an interruption between the workstation and the sage 100 server.

None are on wireless. All computers remain pingable. They all disconnect at different times to each other. If they open, close, and reopen sage, it'll work fine. Only the first session crashes after 30 minutes. Subsequent sage sessions after the first one are unaffected (until the machine is rebooted again)

A few questions which you have probably already considered but I'll ask anyway is 1) Is it happening to ALL users?  2)  Are they properly closing Sage 100 before rebooting?  3) Have you personally experienced the problem yourself, or is it just based on reports from users?

It is happening to all users that reboot their PC every day, with a local Sage client.

I checked how they close sage, and no processes remain after it's gone.

I have not personally experienced the problem, mainly because I don't reboot, and use workstation often.

Since it only happens once, I doubt it is auto-logoff but it's worth mentioning.

I'd suspect anti-virus, with an idle scan happening that doesn't play well with Sage.  There is a KB article on recommended exclusions.