UM 24.04.1 boots to black screen

[tkn]

If you have an AMD GPU, read this:

Radeon GPU prevent autologin + blackscreen at boot

Huh. I was just reading the release notes for the just-released Ubuntu MATE 24.10, when I came across the following which sounds surprisingly similar to what you're experiencing: Switched back to Slick Greeter (replacing Arctica Greeter) due to race-condition in the boot process which results the display manager failing to initialise. I dunno. Maybe you could try installing the Slick Greeter on your Ubuntu MATE 24.04 and see if the autologin works there?

The alternative is to disable plymouth from within grub