Thanks for the report, This seems more like a local network configuration issue than something wrong with factorio, Since you can both connect fine to other servers, and that other people can connect to each of you I would like to clarify that we have not been able to test if anyone else can join o...

Added log files from both parties.
Hope this is helpful.

Loewchen wrote:@OP
The logs of both, the peer and the host of the same connection attempt would be useful.\

I will do my best to get my friends log files but I will not be able to get them until this weekend at the earliest.

Hello. Me and a friend of mine have had a problem with multiplayer in version .14. Whenever I try to join his server it gives me a error message saying: Couldn't establish network communication with server. The same happens when he tries to join my server. We were both able to join someone else's pu...