thx for your help.

What I did:
1. on Server, created a test account (contains the usual demo files)
2. on Client (N900) I created /home/user/oc
(my real account uses /home/user/ownCloud)
3. I edited in .simple_oc.cfg the first three lines
to reflect the new username, password and directory

I called simple_oc
and it worked correctly

4. I put some files into the local documents subdir
and called again simple_oc
and it did correctly upload the new files to the server

ALL GOOD!
But the question remains: why not with my username/password/oc-dir?

Nevertheless thank you so much for helping me with this old little machine.