Asus zenbook 15 (UX-533FD) boot very randomly ubuntu 19.04

In GRUB, press e to edit the linux startup command line and add dis_ucode_ldr at the end of the line that starts with linux /boot/... so that the line will look something like:

linux    /boot/vmlinuz-4.18.0-20-generic root=UUID=63771674-8044-4df4-\
         b700-17bcc857432d ro quiet splash $vt_handoff dis_ucode_ldr

and you'll be able to boot.

(Remove quiet splash if you want to check the system boot logs).

Add the same parameter dis_ucode_ldr to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash dis_ucode_ldr"

then run

update-grub

and then you should always be able to boot, as-is, so without editing the command in GRUB.

See - and if possible, subscribe to - this Ubuntu bug tracker for more information:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1829620