Thanks! It works now!

When a friend tries to connect to my server or to any other, it won't work and it puts out this log:
(He doesn't have any mods installed and it is no specific save game)

0.052 2017-09-15 17:36:50; Factorio 0.15.34 (build 30875, win64, steam)
0.052 Operating system: Windows 7 Service Pack 1
0.052 ...