Thank You for Your help!
You are right telling me that it is at question of accessing the file.
I forgot to mention, that I use an extern harddisk which I connect to my old and my new computer on and off. I can follow Your thoughts, but I do not know how to solve the problem.
I have tried to copy some files to my new intern hardisk on my new computer - and then I can use mp3tag without problems. 
I can see, that the Owner information tells the drive S and a lot of numbers. The files which I can edit shows information on my new PC. Now I have to find out if I can change the ownership as a batch.
Can anyone tell me how to change the ownership?
Regards Jens Solvang DK