there have been reports of difficulty in getting MP games to work since the lastest patch.Flexman said this on www.simhq.com about getting MP campaigns to work
Here's workaround for the screwed multiplayer that may work for you:
1) Run with the command line cvc:0 on both machines.
2) And or try running with command line switch cto:240 also on both machines.
It seems the anti-cheat comms validation is at fault. So the above may help get around it.
Needless to say, a proper fix will be made at a later date.
And this from http://ch-central.frugalsworld.com/main.htm
FOLKS: There seems to be a problem with the SKIRMISH patch! I am not able to fly multiplay at all, and the emails are pouring in saying the same thing.....so don't download it, and hopefully we will hear something quickly from Razorworks.