Might not help but I realised the same monitor working with 2 PC's didn't work with 1 after connecting monitor cable to other PC for a while (maybe the power mode or screen blank / sleep mode for effects reconnecting it or maybe the monitor is somehow only ready for one computer...
Thought I'd mention there might be a difference doing it warmly by moving cable over or between cold boot with cable in. Good luck!
@worldsendless