Hi,
if you don't mind to loose your configuration, but keep all data, then on my 2 disk RAID1 config (J3455B) this worked:
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)
hope it helps.