I could burn the .nrg-file onto a cd, but I wasn't able to install it to my computer (xp). Then I tried to extract the file with CDMage. Unfortunately the only result was "can't open" or "invalid format", so I put all my hope into IsoBuster, and finally it worked!
So, if anyone has the same problem, just install
IsoBuster, open the .nrg-file, select all files in the right window, extract them into some folder and then, voilà, you can install and run LBA1.
No idea why Nero/CDMage couldn't really make the files visible or extract them.
Special thanks for uploading!