Can't boot into Ubuntu 20.04 — EFI partition problem?

Question

I have an Ubuntu 20.04 machine whose main internal drive has 3 partitions:

• /dev/nvme0n1p1, EFI system partition, fat32, 512 Mb
• /dev/nvme0n1p2, unlocked ext4 partition, 732 Mb
• /dev/nvme0n1p3, encrypted LVM partition, 475 Gb

It used to work perfectly, but for a few days I can't enter into Ubuntu. I don't know what happened. Now the PC boots into GNU GRUB version 2.04 (why?), then I have to click for a normal boot and it shows this error message:

Volume group "vgubuntu" not found
Cannot process volume group vgubuntu
Please unlock disk nvme0n1p3_crypt: _

Then I can enter the right password, but the boot always fails a few seconds later. Everything is blocked after a lot of error messages.

I have tried to adapt those instructions and a few others (notably this) to my case. Here is what I did:

• Booted into Ubuntu (“Try Ubuntu”) using an external drive.
• Launched the Terminal app and entered this:

sudo -i

cryptsetup open /dev/nvme0n1p3 nvme0n1p3_crypt (and password accepted)

vgchange -a y
mount /dev/mapper/vgubuntu-root /mnt
mount /dev/nvme0n1p2 /mnt/boot
mount /dev/nvme0n1p1 /mnt/boot/efi
mount -t proc proc /mnt/proc
mount -t sysfs sys /mnt/sys
mount --bind /dev /mnt/dev
mount --bind /run /mnt/run
chroot /mnt
update-initramfs -c -k all

The generated message is update-initramfs: Generating /boot/initrd.img-5.15.0-46-generic.

• Then I type exit and sudo reboot now. But rebooting into the main internal drive shows the same initial error message.

Unfortunately I am not a Ubuntu professional, but I really need to repair that internal drive. Any help will be greatly appreciated.

(Also, is there some repair utility/application that can be downloaded and used from the “Try Ubuntu” external drive ? I would happily try those help apps.)