Repair XFS Filesystem error

xfs_repair /dev/md0
fsck /dev/md1

Check the file system and the re-install the bootloader
Request the rescue-system and then a hardware-reset. Connect to the rescue system.

Possible to mount the RAID?

# mount /dev/md1 /mnt

Yes. Do a file system check (first umount the file system):

# ummount /mnt
# fsck /dev/md1;

Shows some (many) errors. Fixed them by staying on the "y" key. Could use the auto repair option of fsck (-y).
Re-Install the bootloader
This is a Hetzner installimage-setup, so there should be a grub bootloader. Check for the /boot/grub/ folder. Again mount the RAID.

# mount /dev/md1 /mnt
# ls /mnt/boot/grub

It is there, so there is a good chance, that the bootloader is grub, not lilo. Now re-install grub on the disk. Actually on both disks, just in case one is missing.

Make a chroot environment:

# mount /dev/md1 /mnt
# mount -t none -o bind /dev /mnt/dev
# mount -t proc -o bind /proc /mnt/proc
# mount -t sysfs -o bind /sys /mnt/sys
# chroot /mnt

Grub:

# grub

Look for the file stage1 to find the boot partitions

grub> find /grub/stage1
 (hd0,1)
 (hd1,1)

Install the bootloader on both partitions. Both are regarded as hd0 from the point of view of the bootloader at boot time.

grub> device (hd0) /dev/sda
 device (hd0) /dev/sda
grub> root (hd0,1)
 root (hd0,1)
  Filesystem type is ext2fs, partition type 0xfd
grub> setup (hd0)
 setup (hd0)
  Checking if "/boot/grub/stage1" exists... yes
  Checking if "/boot/grub/stage2" exists... yes
  Checking if "/boot/grub/e2fs_stage1_5" exists... yes
  Running "embed /boot/grub/e2fs_stage1_5 (hd0)"...  17 sectors are embedded.
 succeeded
  Running "install /boot/grub/stage1 (hd0) (hd0)1+17 p (hd0,1)...
 succeeded
 Done.

The same for the other disk

grub> device (hd0) /dev/sdb
grub> root (hd1,1)
grub> setup (hd0)
grub> quit

And reboot - works.