boot - Ubuntu Server 12.04 start fails after update

I did an system update on Ubuntu Server 12.04, which requested a reboot.
Since that, the system never reaches the login. It hangs on:

mount: mounting /dev on /root/dev failed: No such file or directory
done.
mount: mounting /sys on /root/sys failed: No such file or directory
mount: mounting /proc on /root/proc failed: No such file or directory
Target filesystem doesn't have requested /sbin/init.
No init found. Try passing init= bootarg.



BusyBox v1.18.5 (Ubuntu 1:1.18.5-1ubuntu4) build-in shell (ash)
Enter 'help' for a list of build-in commands.

(initramfs)

I tried already to repair it using a live system:

• Mounting the filesystems (/boot ext, / btrfs)


• fsck ran without problems.


• /etc/fstab seems to be OK.


• apt update/upgrade on chroot succeed.

Now, I have no more ideas.