OK, i think i found the Problem...
The files I used for installing Loom to HDD (or which i simply copied to HDD), came from a collection of DOS games i downloaded. It seems, that this Version of Loom is a cracked one (It does 't matter, what icons i click in the copy protection, the game starts). I downloaded another Version from archive.org, installed it and the error is gone.
So I searched around a bit to see, what the crack could do to the Game... It seems the the File 096.lfl ist replaced by another. If i deleted this File from the cracked Version and replace it with the original (Not cracked File) the error is gone, but the copy protection is working again... So the Error should be a problem with the "crack".
I have to say, that i have an original boxed Version of the game standing in my retro games cabinet, but no Floppy Drive in my Retro-PC, so i had to download the files somewhere...