Ok, we've tracked this down. We introduced it in our rush to fix the Desura installer issue. Unfortunately we had to work fast to fix that, and didn't have the time under the circumstances to do a full sweep test: it looks like we screwed up Multiplayer Joins.
The good news is we can probably get a fix up tonight on BMT (we can issue the DRM-free BMT key to anyone that needs it once it goes up) - for Desura, we won't be able to fix it till Sunday (because their approval requires someone is there and they don't staff on weekends). Unfortunately that means Indie Royale purchasers won't be able to get up and Multiplaying until then either.
Apologies all, this is pretty frustrating for us (more so for you I'm sure). We're on top of it ... this is unfortunately one of the major disadvantages of being a small indie team working after our day jobs, we don't have all the resources for these situations that we wish we had.