Bela,
the problem is caused by a Samba file "secrets.tdb" which is not migrated during the upgrade process (it's left in it's old directory and recreated newly during the install).
As far as I remember (sorry I am not in the office at the moment), you have to copy /etc/secrets.tdb to /etc/samba/secrets.tdb. Have a search in the forums for this, there should be more details.
Regards,
Michael