How long did you 'wait' in the loading screen before finally exiting SH3?
I had the same problem and waited 'only' for a minute or so. I thought the game had crashed, too. But I couldn't imagine that the GWX team would not have fixed this problem.
So I started another single mission and waited ... and waited ... and waited ...
After 4 or 5 minutes (!!!) the single mission was finally loaded and I could start.
Maybe you should wait a little longer, depending on your system specs.
Be patient, mate!
That's the only advice I can give you, sorry.
Happy hunting!