Hi alcanallen,
It can be caused when the last airbase you flew from in one theatre is not available in another. example if you flew from a US airbase in 1944 then set the game for Battle of Britain you no longer have that airbase available from the pick list. (but temp files in the back end of CFS3 are still calling for it). If you don't go into the location section of QC and pick a new airbase it will cause the game to crash.
to fix the issue go into your application data folder and delete the uisel.xml file. (the game will build a new one)
default path for a XP machine.
C:\Documents and Settings\USER-NAME\Application Data\Microsoft\CFS3 ETO Expansion_________
To stop this from happening I always make sure my last flight is from a base that is in all era's such as Fowlmere in England. before swapping era's on game start up.
hope this helps, regards Rob.