xedit showed a conflict there with Majestic Mountains, which I have ran for ages with no troubles. I reloaded MM but no change. I suspect some sort of landscape fix, or patch. It has been a while since I started the Dawnguard quest, so hard saying when it happened. Haven't ran across any other caves covered up. So, it appears to be just the Dawnguard dlc. I went ahead and got Serana, Everything seems to be working fine. I will play around with it later.