Get the Serious Sam 2 MP demo fix off WP (1mb)
If server was not behind a NAT, or was behind a non-strict NAT, client would get the message "Game session is no longer available" when trying to join the game.
Here is a quick fix for that problem: Sam2Demo-GameSpyQuickFix.zip
It contains two dlls that need to be replaced for this to work. At least the client needs to be patched and the patched client should be able to connect even to unpatched servers.