After HOURS of trying and updating the firmware several times it is now working. OUPGRADE was not helpful at all. I think I managed the upgrade by installing the firmware manually. I think SYSUPGRADE with switches -n -p has made the job.
So I still do not really know what I have done. My next step would have been studying how to set up and configure the overlayfs. Anyway, please check your upgrade scripts. It is very annoying spending so much time for installing a system that should run from scratch.