Strange connection issues [Steam]

[MalDestroyer]

Me and my friend have been playing this Factorio world together for the past week now, and everything has been smooth. Today however, we've been getting connection errors saying "Failing to establish connection to game server" no matter who hosts, me or him. When we try to host a public server for the other to join, only I get a message saying "Failed to determine external IP address. Other players might not be able to connect" even though I know that there have been no changes to my router/internet service for years now. We can connect to other public servers easily so I'm not 100% sure what this issue is and why it's happening now.

[Budnoob]

Hey i cant offer you any fix, just chiming in to say me and my friend are having the same issue since today.

[Kopesss]

Me and two friends have the same issue.
We’ve tried port forwarding, opening the firewall, and resetting the router, but no luck so far

[picklock]

I don't have a solution either, but your problem may be related to issues with Steam networking.

See also a corresponding response from a developer to a similar problem report.
viewtopic.php?p=687404#p687404