When I used Windows 7 on the same PC, I didn't have this problem. Now I can't start my server if monitor is not connected. When I turn it on without monitor, I can't hear HDD loading and also can't connect via SSH. I tried to add nomodeset to my /etc/default/grub, and then grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg, but it doesn't fix anything. Booting with keyboard doesn't work, monitor is a rootcause.
What can I do?