Ubuntu 20.04 Failed to Set MokListRT: Invalid Parameter

You have to replace shimx64.efi with grubx64.efi as follows:

sudo su -
cd /boot/efi/EFI/ubuntu
cp grubx64.efi shimx64.efi
reboot

This is caused by Bug #1867092 and the proposed workaround (see comment #23) is:

apt-get purge shim
grub-install /dev/sda1 #replace sda1 with the correct grub partition for you

That solution worked for me.

Note that the best voted answer to this question, which is copying grubx64.efi over shimx64.efi, will be undone with each shim update according to that bug page.

Solution that works on my 2010 iMac. Install rEFInd and make sure to boot from the option that specifies grubx64.efi.