Yeah, what Farber says.
Awhile back I made the mistake of hitting ESC because the ATAG Server seemed to be frozen when I tried to log in. Looking at that dark screen of London with nothing going on in the loading bar was my downfall. I should've gone down the street to the Williamsburg Pub for a quick pint. Kept getting Steam Authentification Error messages for hours, until a new mission started.
It's a bit annoying, but this is the reason we stop the mission every 4 - 5 hours (sooner if one side achieves all its objectives) and throw everybody out in the snow while we reset.