Recovering a Timeshift snapshot ends with: ERROR: Symbol grub_is_shim_lock_enabled

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@Aragorn Thank you for your advice. Of course I am on GPT. Mostly the success of a search depends on the right phrase for doing the search. Now I looked for “doing a grub reinstall on a non booting system” and I got an answer from AI:

To reinstall GRUB on a non-booting Manjaro system, you will need to use a
Manjaro installation media (USB or DVD) to boot into a live environment.
Once booted, follow these steps:

Open a terminal and switch to the root user context using su.

Identify your system partitions using tools like
lsblk -o PATH,PTTYPE,PARTTYPE,FSTYPE,PARTTYPENAME or
sudo fdisk -l /dev/sda

to locate your root (/), boot (/boot), and EFI system partition (/boot/efi).

Create a chroot environment to access your installed system.

You can use the manjaro-chroot -a command, which automatically detects and
mounts your Manjaro installation.

If this is not available or you prefer manual control, mount your root partition
to /mnt, your boot partition (if separate) to /mnt/boot, and your EFI partition
to /mnt/boot/efi.

Enter the chroot environment by running manjaro-chroot /mnt /bin/bash.

Reinstall GRUB:
For a UEFI system, run:
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck.

For a BIOS system, run:
grub-install --force --target=i386-pc --recheck --boot-directory=/boot /dev/sdX
(replace sdX with your disk, e.g., sda).

Update the GRUB configuration by running
grub-mkconfig -o /boot/grub/grub.cfg.

Exit the chroot environment with exit.

Reboot your system to test the restored GRUB bootloader.

If Windows was recently installed and is now the only boot option, running update-grub
(or grub-mkconfig) after reinstalling GRUB often resolves the issue by detecting the
Windows bootloader.

And I have found another one here on Github:
https://gist.github.com/dianjuar/a6e0d801a9f71f8edc06321178390320

These are the steps that worked for me, without having to install any new packages:

Boot into Manjaro installer

Open terminal

sudo manjaro-chroot -a (and select system to mount)

grub-install /dev/sda (it’s sda for me; make sure you choose the right drive!)

grub-install --recheck /dev/sda

update-grub

exit

reboot

manjaro-chroot is an AUR package which has to be installed on the live-system before.

Do you have any concerns about the solutions, I have cited above?
If not, I will have to choose, how to proceed.