You the man, Ripcord! I'm stuck with Constant Guard, too, since Comcast is my internet provider and that way I get Norton for free (which is a mixed blessing for sure). But I didn't even consider Constant Guard a possible cause of the problem but simply turned off Norton instead. After I read your post I went to Constant Guard, clicked on the Options heading, clicked on Manage my "Blocked Program" rules, and sure enough, it has blocked DCS. I told it to allow DCS to run, and now it seems to be working fine. I've only tried a couple of missions, but thus far they load without difficulty.
Thanks Ripcord and Speed!