I now found the underlying problem, Im on Lp 15.6, but the RPM data base is much, much older from previous installations, that makes the process hang at some point. Thus, this issue affects mostly, or only machines that were setup years ago and carried their RPM data base from distro upgrade to distro upgrade.
The whole issue with a workaround is described there:
https://bugzilla.opensuse.org/show_bug.cgi?id=1242188#c10
The workaround is described in the first posting and consists of converting the rpm database in a 2nd shell, when the upgrade hangs and continue the upgrade after data base conversion, which might take hours! In my case almost 3 hours.