Nervemind...
I am just banging my head on the table right now! The option "user=" in the sessionprofile overrides the given username, of course. I didn't saw it for days.
The credentials are fine without it. I still can't login, sshd saying "Allocating a pty not permitted for this connection", but this is another problem, I think with sshd/pam.
Sorry for the inconveniences!
Best regards
Martin Lange