If somebody faces the same issue. Root cause was a corrupt license file for Plesk 12. In addition you need to verify if you have the Postfix package installed, otherwise Plesk cannot pull a license update.
Hope this might help you.
thanks for your hint but disk has more than 50% free space. I'm getting frustrated now, updated PHP, run autoinstaller with patches and run bootstrap repair. No improvement and no hint in log files. Bad mistake to move to Plesk version 12.0.18 !