BitJam
Our developer fehlix is working on this now in the thread you posted to in the MX forums. He knows a lot more about Grub than I do. As soon as he comes up with a solution we will try to get it to you.
Booting our live-usb via UEFI seemed very solid for a number of years. It even works with 32-UEFI and secure boot. Over the past few days we’ve gotten a number of complaints. Perhaps it’s just a coincidence. From what I’ve read in that thread, we don’t have a solution yet. It seems the problem is related to grub (or our grub configuration) not working with certain machines/UEFIs. So creating the live-usb in other ways (which was my first guess for a solution) will probably not help.
The error message you get is not relevant. It’s not really an error, just Grub demanding for more redundant bloat in a system we are trying to keep small. We can get rid of the error message but then you will be stuck at a blank screen. We can get rid of that error message by making a 4th copy of the same file. We will probably do this.
[...] almost any English person would accept "bully" as a synonym for "Fascist". -- George Orwell (1944)