Okay. try these fixes:
1) delete the snwvalid.dll from your system folder.
2) make SURE you put client.exe, and snwvalid.dll into your outpost install dir.
3) if you have any routers between the computer you will be using to log onto the server, and your net connection, set them so all packets that come on ports 15101, and 15102 go to that certain computer. If you can't route by port, set it so all packets from outpost.sytes.net go to that computer. Also, try shutting off ALL known firewalls.
4) Now open OP2, and click Multiplayer, then the internet (SIGS) button. The client should load after a brief pause. Try to log in as usual.
5) If you still can't log in, the reason could be because you have a dynamic IP, and the ISP changes your IP every time you send data to the net. Other reasons could be that your ISP's routers are confused by the data stream coming from the server, and don't know which computer to send it to. In this case, there is nothing you can do (you might try contacting your ISP to see if they can change it, what you'd have to tell them is "I'm trying to send and receive data from outpost.sytes.net on UDP ports 15101 and 15102, and it isn't working") Other than that, there is nothing you can do.