That does rather imply that the VMG8924 is limited to a single telnet session, and it would seem that an SSH session can be used to clear it. That may be the root of the problem - I'm speculating here, but it could be the case that the telnet session occasionally fails to disconnect cleanly, which wouldn't be a problem with modems which accept more than one telnet login, but leads to a lockup if the modem only accepts a single login.
I've made some changes to add extra checking for disconnects. The program now tries up to 5 times, with a 50 ms pause between tries, and if it fails to disconnect after this, a note is added to the event log.