Failing to run Mint on boot ("Not Found")

I have created an USB key with Mint, but when I boot from it I get a few lines of errors

• Failed to open \EFI\BOOT\mmx64.efi - Not Found
• Failed to load image ??: Not Found
• Failed to start MokManager: Not Found
• Something has gone seriously wrong: import_mok_state () failed: Not Found

I have tried disabling safe boot, and fast boot. I have confirmed that my system has UEFI.

I believe that I once managed to run Mint off the USB but have not managed to do so since