HA fails to boot via USB on RPI 3B

I have a Raspberry PI 3B with an SSD + USB adapter drive that I would like to use for HA.
The system works, as I have been using Raspberry PI OS on that SSD via USB.
However, I cannot get HA to work.
When I plug in the monitor I see a very distorted and hard to read console, and this is what j can decipher from it:

Device 0: Unkown device

Device 0: unkown device
Waiting for ethernet connection... done
BOOTP broadcast 1
BOOTP broadcast 2
BOOTP broadcast 3
*** something option is number
DHCP client something to address 192.168.1.135 (number in ms)
*** no boot? file name: 
Using smsc95xx_eth device?
From server 192.168.1.1: our IP address is 192.168.1.135
Filename 'numbers.something'
Loading TTTTTTTT
Retry count exceeded: starting again 
Retrying file
Waiting for ethernet connection
Using something_eth device?
File name pelinux.cly [pxelinux.cfg?]
Load [memory address I presume]
Loading TTTTTTTT
Retry count exceeded: starting again 
Retrying file
Waiting for ethernet connection
Using smsc95xx_eth device?
From server 192.168.1.1: our IP address is 192.168.1.135
Filename pxelinux.something