Mobo: ASRock H81 PRO BTC R2.0 S1150 Intel H81
GPU's: 6x Sapphire RX570 Nitro+ 8GB
I never had any problems booting the system before as I was setting the rig up whilst the screen was always plugged in and now when everything is set up I must plug the monitor in to boot the system. Never ever had this kind of problem with any other computer systems.
I have a Vega 56 rig (different mb) that has the same problem and I have yet to figure out why it won't boot without a dummy hdmi or a monitor plugged into the gpu that's in the pcie16 #1 slot.
Even using igpu when I need to plug a monitor into that rig fucks everything up.
My 1070ti rigs ... No issues at all with that sort of thing.
Honestly though, for me ... If it's going to take too much time to troubleshoot and you can fix it with a ten dollar dummy hdmi, that's what I'll do. I spent two hours last weekend trying to get igpu to work and it's just not worth wasting any more time on right now. As long as the rigs are hashing, we're in a good mood.