Comment 44 for bug 1842320

I tried the fix with GFXMODE=800x600 -> it did nothing. I had bigger letters but the same error.

I tried the fix with editing mkinitramfs and replacing 1 by 19 on line 196 to restore the high compression level. Then regenerated the image -> it worked.

For readers, this is not so easy, because when you get there your system is stuck. What I did is download an Ubuntu image 21.10. From there, boot on the USB stick. Open a terminal, and:

--------8<-------------------------------------------
$ sudo su -
# mkdir root
# mkdir boot
# mkdir efi
# cryptsetup luksOpen /dev/nvme0n1p3 nvme0n1p3_crypt
# mount /dev/mapper/vgubuntu-root ./root
# mount /dev/nvme01p1 ./efi
# mount /dev/nvme01p2 ./boot
# mount -B ./boot ./root/boot
# mount -B ./efi ./root/boot/efi
# mount -B ./dev ./root/boot/dev
# mount -B ./dev/pts ./root/boot/dev/pts
# mount -B ./proc ./root/proc
# mount -B ./sys ./root/sys
# chroot ./root
# update-initramfs -u -k all
# update-grub
# exit
# reboot
--------8<-------------------------------------------

^ in the above examples, the logical volume names and crypt devices need to be adapted to your setup, obviously.

I do not know what is the gain of switching that compression level from 19 to 1, but the side effect (not being able to boot at all...) is quite annoying.

Thanks very much @ybdjkfd for the fix, that saved my day (if not my week & month).