There can be issues that exist between particular UEFI display output configs and certain monitors. I've had issues in trading floor support with HP Z workstations and both basic Dell 22" screens and large Samsung ultrawide 34". Something again recently with a Z series workstation and big 2x 48" screens (Dell I think). I don't know the specifics of what this is but the solutions are: A) Run in legacy mode B) Boot up with the display cable removed and then put it back in once the system has reached the OS so that it doesn't run into the UEFI conflict C) If you have onboard graphics, add in another graphics card. This isn't helping the issue, what you'll still see is a blank screen during POST and inside the UEFI settings screens (so you need another monitor if you ever actually need to access these things), but it might be enough to stop machine from calling a POST/boot failure and freezing.
Warning: I removed the heatsink and now that I’ve re-applied the paste it no longer makes any contact at all with the graphics substrate and barely any with the CPU. HP have a habit of splurging several mm of paste onto their CPUs during manufacture and this has either bent the heatsink or it’s just a poor design full stop. I guess I’ll get a thermal pad and use that as it’s at least designed to conduct heat probably better than an ocean of thermal paste would
