Hi MotoX80, thanks for answering.
Firstly, sorry for the screenshots in German, but you should be able to understand anyway.
I can see my shared machines:
When trying to lock-in first I get the normal lock-in mask:
When entering the Microsoft account credentials I am getting this answer:
It says that either the username or the password would be wrong.
When using the terminal I am getting another error message:
Here it says: Access denied
It does not matter if I enter the name and password of the local user or of the Microsoft account.
Strang enough I have another machine in my network wich I can access from all machines without entering any credentias. I cannot figure out where the difference between this machine and all others is.