What about firewall? Do those 'other' users allow activity of your application? I can confirm what Michael_Schwarz said. I have had this problem too and spend hours thinking my program didn't worked. In the end it was just that I had to connect from localhost if i'm on the same connection as the server.

My friends could login from the internet after i'd portforwarded to the server on my LAN. There were some problems with ports (bug) and firewall issues that prevent the application from working correctly at first.