Booting from ISO suddenly stopped working

(Linux Mint 19.3 XFCE w/BTRFS; ISOs are on an EXT4 partition)

I’m trying to get my system to boot from ISO images. I added the following entries to /etc/grub.d/40_custom:

submenu "System Maintenance Utilities" {
    menuentry "Clonezilla live" {
        insmod part_gpt
        insmod ext2
        set isofile="/clonezilla-live-2.6.7-28-amd64.iso"
        set root='hd0,gpt5'
        loopback loop $isofile
        linux (loop)/live/vmlinuz boot=live union=overlay username=user config components noswap edd=on nomodeset ocs_live_run="ocs-live-general" ocs_live_extra_param="" ocs_live_batch="no" locales=en_US.UTF-8 keyboard-layouts=en_US vga=788 ip=frommedia toram=live,syslinux,EFI findiso=$isofile iommu=pt
        initrd (loop)/live/initrd.img
    }
    menuentry "Gparted live" {
        insmod part_gpt
        insmod ext2
        set root='hd0,gpt5'
        set isofile="/gparted-live-1.1.0-5-amd64.iso"
        loopback loop $isofile
        linux (loop)/live/vmlinuz boot=live config union=overlay username=user components noswap noeject locales=en_US.UTF-8 keyboard-layouts=en_US vga=788 ip= net.ifnames=0 toram=filesystem.squashfs findiso=$isofile iommu=pt
        initrd (loop)/live/initrd.img
    }
}

This was working great for about 3 days, then it just stopped. I have not done any upgrades nor major updates. I added the exact same entries to my Mint 20 VM and both worked as expected. The errors reported when I try to select either ISO boot are:

error: invalid sector size 0
error: no server is specified
error: you need to load the kernel first

It does not matter where I put the ISOs. I know that the ISOs are being seen as if I change their names to garbage, my system complains about them being missing. I’ve tried using Timeshift to go back one week ago, when everything was working, but nothing changed.

UPDATE: even though my PC is running GRUB v2.02, I tried the fixes in the link provided – nothing changed (the module "tpm" was not found). I’m now at the point where the ISO is not even being accessed; I can put in complete garbage for the isofile variable and the errors are the same. I even tried reinstalling GRUB, with no effect.

UPDATE2: Smartmon extended tests come out with no errors. I tried moving the Gparted ISO to another disk (EXT4 partition where my /home lives); now it can’t find the ISO at all, no matter what settings I put into /etc/grub.d/40_custom. I reverted it back.

UPDATE3: Just to be safe, I used these ISOs to build bootable USBs – one with Linux Mint (using Fedora Media Creator), the other with Windows (using AIOBoot). All 17 of my ISOs worked.

UPDATE4: Just completed a fresh install of Mint 20. The issues still remain.

UPDATE5: Definitely leaning towards a hardware issue. I cannot boot either Clonezilla/Gparted ISOs from my current system, whether from a local partition or a multi-boot USB. Distro ISOs are still working – Mint 19/20, Ubuntu 20, Windows 10 tested.

UPDATE6: I tried reflashing my BIOS after I finally got a USB of Clonezilla to boot…but it got my 2 SSDs reversed!