Kali: Installed & Working but GRUB/Boot Stuck on Upgrade

Good morning: Some time ago, I managed to install Kali on my computer and, following some more experimenting, got it going. But I suspect that there might be multiple GRUB installs on the computer or some other problem. Kali has worked all right for months but after today's upgrade attempt that got stuck at 96%, following these error messages, I would like to resolve the Boot/GRUB problem. How can I clean up my Kali installation and successfully upgrade it? Thank you.
grub-install: warning: Cannot set EFI variable Boot0000.
grub-install: warning: efivarfs_set_variable: writing to fd 7 failed: Interrupted system call.
grub-install: warning: efivarfs_set_variable: failed to unlink /sys/firmware/efi/efivars/Boot0000-8be4df61-93ca-11d2-aa0d-00e098032b8c: Invalid argument.
grub-install: warning: _efi_set_variable_mode: ops->set_variable() failed: Interrupted system call.
grub-install: error: failed to register the EFI boot entry: Interrupted system call.
Failed: grub-install --target=x86_64-efi
WARNING: Bootloader is not properly installed, system may not be bootable
Generating grub configuration file ...

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libor@hostname:~$ lsblk -f
NAME FSTYPE FSVER LABEL UUID FSAVAIL FSUSE% MOUNTPOINT
mmcblk1
├─mmcblk1p1 vfat FAT32 B333-AD2F 510.6M 0% /boot/efi
├─mmcblk1p2 ext4 1.0 4d3cf90a-ffd4-4d4a-90f6-972ddba98c9e 2.2G 91% /
└─mmcblk1p3 swap 1 ae3a12d8-42fe-4f07-96d4-c500c052941d [SWAP]
mmcblk1boot0
mmcblk1boot1