Solution:
It is possible to recover the damaged slot from the working one.
First identify the working slot (A or B), and use it to recover the bad one.
For that, in the GRUB menu boot into one of the rescue shells (options 3 or 4), and then using the “dd” command clone the working slot into the damaged one.
Each slot have 2 partition (Kernel and file system) as following:
SLOT A
Kernel - /dev/sda2
FS - /dev/sda4
SLOT B
Kernel - /dev/sda3
FS - /dev/sda5
So, for example if slot A is damaged, you need to runs the commands:
dd if=/dev/sda4 of=/dev/sda2
dd if=/dev/sda5 of=/dev/sda3
Reboot, and both slots are now working.
Credit for the solution to @mbcomer who posted it on Jan 27th: