Itd, thanks for posting this question and Ray thanks for your response.
My issue was very similar except my Pi kept rebooting due to a low voltage warning. My display was showing the boot process and then the spontaneous reboot at a random point during the reboot cycle. Like Itd this only happens when the board is powered via the 24v supply. My board has been running fine for over 6 years and like Itd my OSPi board showed no obvious signs of failure. so I am not sure what has caused the sudden issue.
I used your workaround, although the first time I connected the commons I made the mistake of doing so through the green plug and plugging it back into the board and was surprised when my Pi started to boot. Interestingly it actually booted all the way to a login prompt but I did not trust it. I re-read your instructions and realized you are supposed to connect the commons without it being connected to the board. When I did that I could use an USB power supply for the Raspberry Pi and it is all working fine.
So thanks for the great support.
All the best.