SureAI

Ok, I've looked around the forum and found no trace of this problem with anyone else. So let's start from the beginning.

First time I started, the mouse pointer and all the submenus were white boxes. I also had the "New Game"-bug, but found the solution. Now the game crashes every time I try to start it instead. It crashes as soon as the loading bar shows up.

Before I've had a bug with Oblivion that happened whenever my Data folder got to full. The game simply didn't load some textures, and eventually the sound fell away, and then finally the game crashed. The solution was to remove files from the Data folder. My Nehrim Data folder is much larger. I think it was file numbers, rather than file size, but the bug with white boxes in the beginning certainly made me suspicious. What should I do?

To me that sounds like your original oblivion data files have been overwritten by mods. now nehrim doesn't find the files it needs.
Try a clean reinstall of oblivion, latest patch and then nehrim as described in the readme.

Oh great. What files are the vital ones? I have some backups of some of the files. I assume they are bsa-files.

EDIT: I think I have fixed the problem now. I had unpacked the BSAs for modding purposes and then moved them out to avoid the bug I mentioned above. By simply copying them, I fixed the problem.