RX480 rigs wont boot without monitor connected

As the topics say.
I now have two RX 480 rigs running ETH but none of them will boot without a monitor connected to the main gpu.

The rigs are running Windows 10 and they can run ultra stable for days without any problems. But when i reboot the systems they will halt before windows start loading. As soon as i connect a screen and reboot the system again they fire up right away.

Anyone else having this problem ?

Rigs spec:
Asrock H81 BTC Pro
Dual core Celeron
4Gb DDR3
Silverstone Strider 1200w
SSD
Powered USB risers
Reference RX 480 8GB