None of the above solutions worked for me (reinstalling UE4/project/updating drivers/setting NVidia gfx to default).
What did work: When I plugged in a second monitor, the error went away.
It feels like it had saved some info from a prior session where there was perhaps a window on a second display? By my experience, the issue seems very likely to be something saved in the UE4 engine or project .ini files and/or registry settings as I wiped out just about everything else in my attempts to clean things out.