I've recently (2-3 weeks ago) upgraded my Ryzen 5800x based PC, from an Nvidia card to a RX6900XT, and initially everything worked fine.
However, suddenly I now have an issue regarding my secondary monitor (Asus VG248QE connected via DP) going black with an on screen message "Displayport no input". No drivers were updated or changed, afaik.
It seems like the monitor is now forced to a resolution of 1280x800, which it does not accept, hence producing no image. I can see that windows desktop believes that I have two active monitors, my main (Alienware 2518HF connected via HDMI) running at 1920x1080, and the Asus VG248QE supposedly running at 1280x800.
I am running in "Extend Desktop" mode, and I've noticed that moving the mouse to the desktop on the secondary monitor works fine, so windows is pretending that the secondary monitor is active.
I've tried reinstalling Windows 10 Pro x64 again to have a fresh start, installing only motherboard drivers (Gigabyte B550 Pro V2) and Radeon Adrenalin 21.6.1 (have also tried with Radeon Adrenalin 21.8.1). Same issue. I tried the standard PnP monitor drivers at first, but that had no effect, nor had upgrading monitor drivers/inf files.
I also tried changing the port on the card, and setting a custom resolution via Radeon Software - but that didn't work either.
Next steps will be attempting older Adrenalin drivers... I will post my results here.