For reasons still unknown to me, it seems that sessions are ending prematurely.   They should be ending after 12 days of idle (or forcibly after 360 days)... but it seems they are instead ending after around 2 hours.

This is a bit of a troublesome issue to find, so it may take some time.  Please be patient.

UPDATE:

Actually, this was a pretty easy fix.  Just had to fix some old code.