I always got the annoying Microsoft Visual C++ Runtime Error R6025 - Pure Virtual Function Call after visiting Soul Cairn during the quest Beyond Death in Dawnguard which crashed my game after 2-30 minutes play time. After trying many trial and error tests, I finally found the cause: a broken script of Soul Cairn Horse Quest that wasn't shut down properly. I hope this small fix would save anyone who has the same problem from the frustration I had with the DLC for so long.

Note: This fix can only prevent the problem. If you have already done Soul Cairn Horse Quest and got the error, this fix can not help you. You need a save from before starting the quest.

Credit: Bethesda for making such a small mistake that gave me a headache for year.