When I try to enter the cave I get a black screen and the mouse starts to lag around and nothing happens. I can only end task with Task manager. Tried verifying game files, didn't help. Anyone else have this problem?

I had a similar problem. But I got in to the cave after waiting for a while. There is something unfinished about that place, but you are not missing anything loot or story wise. Its kind of cool that the entire cave is completely dark and you cant see anything without the light spell or a torch thought.

Could you explain more? Like how long, in real life or in game resting?

It might have taken a minute or two of loading. I have an SSD though.