From what I could gather, for stand alone, the times it would run would usually be after I did a clean up and repair, then it will work (or maybe not) on the first run but subsequent tries it won't.
Steam edition worked after once after I reverted to stable, then back to beta and another time after the hot fix went in on 22 feb.
Other times I've reverted back and forth with no luck.
When 2.5.6 first went live I had trouble with my virus scanner (McAfee) but once DCS was whitelisted it didn't appear to interfere. I uninstalled the virus scanner completely then uninstalled and reinstalled DCS (steam). I've since reinstalled the scanner but if need be can delete it again.