thanks very much for your help!
Actually this method worked for me:
WARNING: you'll loose your configuration and installed apps!
create the syno pendrive from scratch (it's corrupted during upgrade)
create an ubuntu live pendrive
boot up linux and open terminal
sudo -i
apt-get install mdadm lvm2
mdadm -Asf && vgchange -ay
fdisk -l | grep /dev/sd
mdadm -AU byteorder /dev/md0 /dev/sda1 /dev/sdb1 (and list all your first partitions of the disks)
mkfs.ext4 /dev/md0
now all your 1st partitions are formatted, reboot removing ubuntu pendrive and inserting syno pen
choose reinstall
use manual install and select last working PAT
wait....., and you'll be DONE (of course all apps and configs are lost, but data should be there)