Well, I am having the same issue, I'm afraid. I cannot, for the life of me, get to the adventure after "Coronation Day." After beating that mission (for what must be the 12th time now), I speak to Darius and get the "3 Years Later" text. The screen then shows the Leopard-class Dropship flying toward a planet, the music plays, the "busy" circle in the upper-left moves and moves, and the advice to play the game continues to display messages...
...but that's where it stops.
I have tried disabling my Anti-virus program (though I can't tell if it is blocking BattleTech; there does not seem to be a screen for that).
I have changed from full-screen mode to windowed mode and back.
I have changed resolution several times and mixed both aforementioned viewing modes.
I have restarted my computer multiple times (yes, it exceeds the required hardware).
I have reinstalled three times.
I have also attempted these things with Steam in offline mode.
No matter what I try, the game will not continue past that point. The crazy thing is, it seems to play flawlessly up until that point.
Any ideas, fellow gamers?