I found a workaround for this. I put an EFI shell on the boot partition in the directory EFI/boot and named it BOOTX64.efi .
Now, the firmware doesn't change the boot entry any more and booting works.
Multiple different boot entries still don't seem to work.
Hello WJoost, you may want to forward your inquiry to Linux* distribution support community for further assistance since this is an operating system that we do not support.
I really apologize.
I know Intel doesn't support linux on this mainboard. But I hope you support the firmware of the board.
I just had a look into the UEFI Specification 2.4 from Juni 2013. In section 22.214.171.124 is written:
"[..] There may also be an optional vendor subdirectory called BOOT.
This directory contains EFI images that aide in recovery if the boot selections for the software installed on the EFI system partition are ever lost.[..]"
So, a file \EFI\BOOT\BOOTx64.EFI (or BOOTIA32.EFI) is optional. However, the board firmware requires \EFI\BOOT\BOOx64.EFI to be present. Otherwise, it invalidates the BOOTxxxx-entries. In my eyes this is a violation of the EFI standard and a firmware bug.
It looks like Windows (at least Windows 2012) always puts a \EFI\BOOT\BOOTx64.EFI file on the EFI partition and therefore can boot.