So are users going on sage via the server or via their own workstations.
From the look of the error, I'd guess they were staying logged onto the server at night leaving sage open, not closing it properly creating stale sessions.
Some scanning over the internet
The issue is definitely a problem with a piece of software running upon startup and confusing the network configuration.
Not sure what that means, would think the server is looking for sage updates or something, sage should be per machine software wise, nothing more than mear speculation from me.