I found the reason for this bug :D If you ever had installed the mod "flowing cape" and changed the .ini settings, but forgot them after uninstalling the mod, the game crashes every time you want to enter the volkihar keep. You can read it here: http://forums.nexusmods.com/index.php?/topic/933400-persistent-crashing-upon-entering-castle-volkihar-spoilers-inside/