Did you manage to solve this issue? I'm having the same problem only with 2 monitors connected. One on DP and another one on HDMI...only the HDMI goes grey tho. To fix it I'm just using windows+P -> main screen only and then back to "extend" screen again. Not ideal tho.
The HDMI monitor it's a DELL P2311H with a VGA -> HDMI adatpor actually.