[solved] I want to install MX-18.3 to partition #19 on my HDD, but...

[fehlix]

The list-new had two keys for openSUSE, the list-enrolled was empty. Resetting did clear the list-new, without needing mokmanager. The problem still persists, except with the suggested .efi workaround.

I fear it's a bit more .. you probably still see the keys with
mokutil --list-delete
which is "List keys to be deleted"
And this is the crucial bit: The actual process of adding/deleting/revoking keys is done during boot through shim/mokmanager combo.
As long as there is something not "finished" or marked as to do, the process at boot with UEFI triggers shim to call mokmanager.

[jbMacAZ]

While --list-delete also came up empty, loading a Ubuntu liveUSB did invoke mok. It apparently was sufficient to run it even though I didn't know what password it wanted. Anyway, this solved the issue, and now my Debian liveUSBs boot without manipulating the ESP .efi files. Thank you so much.