Whether you're trying to play a MultiPlayer game by yourself or with others, in Stock v1.5, look under your game screen after creating the session, and see if you don't have a Windows' Firewall dialog that is wanting you to grant access to "Silent Hunter IV". That's why it quits - the session times-out. The UPlay does not work, but you can do private network LAN play. I'm not familiar with the mod itself though...
