On some server or workstation machines you may encounter the situation that XenServer or XCP-NG will not boot after installation if no display is connected. In this case, it was a Lenovo ThinkCentre workstation in our lab, but we also heard about some Fujitsu servers affected by this.
The solution for this is pretty simple, but may have a side effect. First connect a monitor and boot your server. Start XenCenter, go into the properties of your XenServer host and switch the GPU option to “This server will not use the integrated GPU”. This basically disables graphics related stuff on the XenServer console, including the boot splash screen. Reboot the machine. XenServer / XCP-NG will now boot without any display connected.
One side effect we experienced on a machine was that the text UI was gone to, the console output was blank. As management is usually done via XenCenter or XenOrchestra, this may only matter in case of any hardware problems. Be aware that all changes at your system are done at your own risk. We also haven’t tested the impact of the GPU option in regard to the graphics power of the virtual machines.