Ok, the suggestion with the port-randomization made me think a bit. I then tested with a few friends how my UDP-packets are getting changed by my router. It seems that my router changed the source port of the packets to some number in the 60000 range. Why it did this I have no idea. I then removed t...

I disagree that this is an error on the client side (i.e. configuration of network gear). Not being satisfied with doing nothing I fired up Wirershark. I filtered for traffic only from and to the two pingpong-servers (pingpong1.factorio.com // pingpong2.factorio.com) and tried to host a new multipl...

Same here:

The port was never changed, it even says in the log file:

33.150 Hosting game at port 34197, peerID 0, session magic 1054

And in config.ini:

port=34197

I tested every version from 13.3 to 13.5 and on all versions the same error occurs.

I want to add that the same error occurs if you want to use one of the following letters ö,ä,ü (german umlaut) or è,é,ê (french accents).
Could it be the case that only ASCII is allowed?

Whenever I try to create a multiplayer game (regardless of the settings like private/hidden/etc.) I get the following notice: Failed to determine external IP address. Other players might not be able to connect. Looking at the log I found that the pingpong server did reply with the same IP address, b...