Fewer screens connected than expected
What it means
A backglass or DMD component is set up, but fewer screens are connected right now than it expects.
Why it happens
This only counts what's currently connected — a cable unplugged, a monitor asleep, or displays reconnected in a different order can all cause this, without anything actually being broken.
How Pincab Toolbox detects it
Fewer than 2 monitors are currently detected as connected, while a multi-screen component (backglass, DMD) is present under the install.
Check your own pincab
Pincab Toolbox is a free Windows scanner for Visual Pinball X and PinUP Popper. It finds and explains problems like this one. No account, no telemetry.
Download for freeCovered: Visual Pinball X (10.x) and PinUP Popper. Not covered: Future Pinball, Pinball FX, Visual Pinball 9 and the other frontends (PinballX, PinballY).