You need to do some research before messing around making random changes.
You may want to try and uninstall grub-customizer, of course....
According to the blog I mentioned before:
"Grub Customizer has put the entire contents of /etc/grub.d into the backup folder".
Since /etc/grub.d is not deleted at the uninstall, you could try to restore that back-up and retry the upgrade-grub.
Not sure what the blogger meant after with " The same with /boot/grub" though
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Clean your system from grub-customizer files, re-install grub and update grub. (using chroot from ISO installer).
Read another case
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