So to close this thread on a positive note, logging out of the account in Factorio's options ("Other" section) and then relogging back fixed the issue, I can now load saved maps and the error is no longer popping.

Yes, same exact error as you. We played last week without any issues. Posting my logs would, I believe, show nothing more than what you've already shared, I'll let the devs work on it since this is related to the game itself and not local/router settings. We went from versions 15.34 to .35 and .36 ...

Hi, we've been playing Factorio for a few weeks now (3 friends on Steam), but as of tonight, we can no longer play together, when loading our saved game as the host it says "Failed to determine external IP address. Other players might not be able to connect." Once started, when friends search Steam ...