I think i go a bit closer to my problem with the login via a display
manager: I tried several login managers, and with allo of them trying
to login as user brings me into nirwana.

Now, i tried to login as root (+ passwd) via Slim and voilà, it worked!

So, my suspect here - but without any idea how to "repair": since i had
to do a downgrade (to be able to use the fglrx driver!) from ascii to
jessie, i did a fresh install, but i left the /home untouched by the
partition program of the installer (although i redefined it as /home)
because i wanted to keep the user settings i did before. Now, the home
works yes, but may be there is something that should be done in
addition to get it completely integrated?

When i do startx the .xinitrc line 'exec: jwm' (or startlxde or
openbox) is read correctly and X is started with the respective desktop
environment.