Installer can't add EFI boot entry

[aeonoea]

So with a few old Intel Mac minis lying around who make excellent servers, I thought giving xcp-ng a spin finally, but encountered an efi error.

Hardware

• Mac mini 2014
• SSD (orignal)
• HDD (original)

More details:

• No fusion drive, seperate drives completely wiped before
• No secure boot (introduced in 2018 model)
• Other OS like Proxmox, Win, Linux etc. can run and install just fine also via EFI boot

Issue

Installer won't complete fully. At around 50% of the "completing" phase, it would return an unrecoverable error:

Failed to add new efi boot entry: Could not prepare
Boot variable: Invalid argument.

Interestingly after confirming the error, xcp-ng would just boot up fine, but is useless as the root password is not working, as already experienced by another user here.

Starting the installer with the alternative kernel didn't make a difference unfortunately.

Traceback

Traceback (most recent call last):
File "/opt/xensource/installer/install.py", line 284, in go
backend.perfornInstallation(results, ui, interactive)
File "/opt/xensource/installer/backend.py", line 476, in performInstallation
executeSequence (fin_seq, "Completing installation...", answers, ui_package, True)
File "/opt/xensource/installer/backend.py", line 283, in executeSequence
updated_state = item.execute (answers, progressCallback)
File "/opt/xensource/installer/backend.py", line 81, in execute
rv = apply(self.fn, args)
File "/opt/xensource/installer/backend.py", line 1125, in installBootLoader
setEfiBootEntry(mounts, disk, boot_partnum, install_type, branding)
File "/opt/xensource/installer/backend.py, line 1185, in setEfiBootEntry
check_efibootmgr_err(rc, err, install_type, "Failed to add new efi boot entry")
File "/opt/xensource/installer/backend.py, line 1157, in check_efibootmgr_err
raise RuntimeError ("%s: %s" % (err_type, err))
RuntimeError: Failed to add new efi boot entry: Could not prepare Boot variable: Invalid argument

(may contain some typos as transcribed from a photo)