Apparently there is a problem with upgraded Windows 10 systems which has a corrupted default user file. Creating a new user works fine but a logon leads to a “User cannot be loaded” Error.
Windows tries to create the new user profile by copying the necessary files from the default profile (which is corrupted) and fails doing so.
This can be solved by overwriting the Default Users’
C:/Users/default as shown in other tutorials. My Problem was to obtain a non-corrupted user file.
So to help everyone with the same problem, one can find an working default user file HERE!