I had the same problem, I reformatted my harddrive, reinstalled DCS and still had the same problem. I turned on User Account Control after turning it off and then right clicked the DCS icon and forced 'run as admin' then I played a mission, unchecked "fullscreen" and all of a sudden the training missions started to work, I don't think anyone has nailed down the answer to the problem, but I won't say go as far as reformatting your harddrive, but do a fresh install and make sure all your old folders have been erased. Then fly a couple missions and check and uncheck "full screen" along with running the program as admin.