Can't boot into Live USB, though secure boot has been disabled

I also have an Acer Aspire E5 and had the same problem. Follow these steps:

They assume that you are installing Ubuntu, but might also be required for booting from a Live USB

1. Make sure your UEFI is up to date.
2. In UEFI, you must set a supervisor password. Once the supervisor password is set, some new menus are accessible.
3. Enter the "Select an UEFI file as trusted for executing" menu. It will allow you to declare the newly installed Ubuntu boot image as trusted on your laptop.
4. Select EMMC >> EFI >> Ubuntu and select the file shimx64.efi. You will be prompted to give it a name of your choice (which will appear in the F12 boot options).
5. Type the name press Enter and then "Yes" should be highlighted.
6. Press enter again. Restart the computer without any boot disk and tap F12 during the boot process.
7. You should now get the boot option menu with the name you chose.
8. You can change the boot order in UEFI now so that it boots directly to the Grub screen.
9. Either disable the supervisor password, or save it in a secure place.