Ubuntu 18.04 on Mac Mini mid-2007

I am trying to install Ubuntu on a mid-2007 Mac Mini (Core 2 Duo 2.0GHz). I received the computer second-hand and it has Linux Mint installed, but I don't have the admin password, so I want to do a clean install.

I am using Windows to make the USB bootable drives. I have followed the instructions in Spade's reply here:

Ubuntu on a (mid) 2007 Mac Mini?

1. I can get rEFInd to boot on the Mac from a USB.
2. I formatted a second USB as GPT using Diskpart.
3. I downloaded the Ubuntu 18.04 iso and burned it to the second USB using Rufus.
4. I then added the bootia32.efi file to the /EFI/boot folder.
5. I plugged both USBs into the Mac Mini and booted while pressing Alt/Option.
6. I selected the rEFInd EFI boot option and landed on the rEFInd screen.
7. One of the options was the orange Ubuntu icon. The label below it says "Fallback boot loader from Ubuntu 18_0". I selected that.

At this point, there is a black screen with a blue box that says "rEFInd booting" and a very fast flash of an error message (it's too fast and I can't make anything out except the word error). Ubuntu doesn't boot, however, and I don't get GRUB. It just goes back to the rEFInd screen.

I have installed Ubuntu on many computers before, and have gotten it to run on modern Macs, but I've just done it by following instructions and have very little experience with the guts of the system. I just want to make this Mac into a usable system and I don't know what I'm doing wrong.

The GRUB image on the Ubuntu ISO is EFI64 only. You need to create an ia32 installation of GRUB on that boot disk to be able to boot in EFI mode. Thankfully, Fedora and Debian ISOs include the needed binaries, so you can copy bootia32.efi and /boot/grub/i386-efi from them. Once installation is complete, there is no more tinkering required with the installation: the required GRUB files are automatically added by the installer.

Follow this answer here, but use the disk image hosted over here instead of the broken link. You don't have Secure Boot, so you can ignore that step. You should also use GNOME Disks to change the partition type to EFI System Partition, and create a GPT partition table rather than MBR.

Alternatively, whilst your Mac can only load EFI32 images, its legacy BIOS emulation mode will load 64-bit bootloaders. Therefore, you can try and remove the EFI boot partition to trigger it to read the disk as a BIOS boot mode disk, or select the 'Windows', rather than the ‘EFI boot’ option from the boot device selection menu if it is offered.