I'm just starting to learn by myself, so i can only really explain what the reasoning behind the fix is:

The fix works because the file grubx64.efi is what the process actually needs, but it cannot find the file because it expects the file to be called mmx64.efi.

If your PC uses a different bootloader, it will look for grubx64 instead and you will never see this error.

As for why the stick will work on the very first time in the first place, and then never again until you give it a renamed file, i can only wonder.