Tonight I reinstalled my system with openSUSE-Tumbleweed-NET-x86_64-Snapshot20141201-Media.iso and the installation went flawlessly.
But now I made it slightly differently. Now there was an existing correctly bootable system on my disk.
At first I did not provided the password when the setup asked, just in the expert partitioning settings after requesting a disk re-scan.
Then I formatted the /boot and the /root and the swap. And to my surprise after the installation my system booted.
(Plus beforehand I just prepared an unformatted (0x83) primary sda1 for boot, and a primary sda2 LVM2 (0x8E) partition that I encrypted with cryptsetup luksFormat. And later in (the setup) yast I formatted the sda1 to ext4 and set to /boot, and on sda2 created a volume group, then the logical volumes with formatting and mount points.)
I suspect: because my disk wasn’t completely empty yast red my settings from the /boot (from initrd?) and/or from the /root before the formatting. But it is possible I’m wrong here; I don’t know.
Maybe the constellations… the Saturn and the Moon… you know… :P:quiet:
Unfortunately currently I don’t have more time for further investigations and experiments.