I would agree it certainly ~sounds~ like a firewall issue. The computer having comm trouble is behind a home modem/router, trying to talk to other peers behind the same router. I don’t have a proxy. Just a few computers NAT’ed behind the modem/router.
On the box itself, with Zonealarm firewall off (I don’t use the Windows firewall), I don’t know how to make the firewall do any less than turning the whole firewall off. The issue was fixed by using v.41, so if you are correct, it seems to be a firewall issue only with the newer v.42 version. I’ll test the whole thing again when the next version comes out.
However, matching your suggestion, I had already manually deleted all entries for syncthing and re-allowed syncthing to do any inbound and outbound comm it desired. That didn’t work, which is why I just turned off the entire firewall.
(BTW, remembering to our other conversation about staying always up to date with the newest and the best… I’ll point out, that this problem started only when I did allow an upgrade. Experiences like this are one reason I have a bias toward older stable software and architectures.)