The problem with the "rothoff" login isn't that it goes to the wrong directory, but I can't log in at all with that login.
But adding admin rights to a client's users/email did allow me to get in and update the web site. Which is what I needed.
I'll try to find out the reasons whey admin, resellers and clients can't log into the server for FTP, even though they have FTP access.
As I always say: "Get things working first, THEN improve them." I got it working now...