Thanks Neoauld. That's definitely proving to be very helpful.
Still, it would be nice (if this is technically feasible) for Stardock to have more specific error messages in Demigod, or just have some kind of P2P diagnostic tool. It would help to be able to tell if closed ports are the problem, or if my ISP is filtering P2P traffic (although they deny it, I suspect them of doing so), for example.