Hello I have a Retina MBP 2015 15" that is connected to an external HP Z27n monitor via DisplayPort. When my MBP is on my desktop I connect it to this monitor and close the lid (clamshell mode). Everything works ok when I am inside macOS, but if I have to restart the Mac while the external monitor is connected there is a point on the start process where the external screen is going black. I get a message on the monitor that there is no signal and it goes to sleep. If I open the lid of the Mac and turn the monitor off and on it starts to receive a video signal again and I can close the lid. The thing is that at the beginning of the starting process everything is working fine. I get the Apple logo and then the screen to input my credentials (I use FileVault). The OS then continues booting and I can even see the desktop loading with all the icons, but then suddenly the external monitor goes black and I have to do what I have posted before to get the picture back. I have changed ports in the Mac for connecting the DisplayPort cable but nothing changes. I have played with the settings on the HP monitor without success either. Anyone has any idea on what can be happening here? I do not restart my Mac everyday but when I do it this is very annoying. Any help will be greatly appreciated. Thanks.