Hi first time poster - and first time usage of Unraid. I'm experiencing similar behaviour when booting....fails repeatedly (when headless?). Boots successfully when a monitor and keyboard attached following entry/exit into bios.
Version 6.11.3 did not boot at all as a fresh install (known bug). So using v6.11.1.
I have tried various branded usb sticks, used the installer (windows) though found manual install to usb more reliable.
Up and running for now and enjoying the Unraid experience.