There is a procedure written by @mladen to recover a non booting device: Can't use L5 after update!
It seems that the steps #10 and #11 of this are outdated.
- Delete “initrd.img” and “vmlinuz”
- Rename “initrd.img.bak” to “initrd.img”, rename “vmlinuz.bak” to “vmlinuz”
and should be updated by someone from support@