From memory there was a October expirey on the Beta, maybe that wasn't removed. I rolled back both computers to yesterday to restart te Stardock Multiplicity service, and it is running fine.
Okay, both of my machines that aren't working (the win7 and XP) had the Beta installed, although I fully uninstalled the beta and installed the 'official' release (not just an 'upgrade' install) as soon as M2 was out of beta. The Win8 machine that's working fine was a fresh install of Win8 and a fresh install of M2 (no beta install, in other words).
This is beginning to look like the culprit.