Hi,
I followed above advise (thanks for posting that) and removed the 2 registry keys. After that I connected the screens one by one. Windows 10 initially named my playfield screen 1, backglass 2, but after plugging in my dmd monitor, windows numbered the backglass 3 and the DMD 2.
So I than physically moved both screens to the other ports on the graphics card (I switched backglass and dmd) and started over. Now Windows has them numbered in the correct order:
playfield = 1
backglass = 2
dmd = 3
my setup looks like in the screenshot attached. I configured windows to "extend the screens" on the 3 monitors.
I configured PBX with the same settings, and gave the monitors there the same number (see screenshot 2). This initally worked when i tested it.
But, after a reboot, pinballx changed the order of backglass and dmd again (sometimes!)... displaying the dmd on screen 2 (backglass) and the backglass on screen 3(dmd) as you can see in screenshot 3.
After I made that photo, I just rebooted again, without touching any config files and as you can see in screenshot 4, pinballx displayed everything again on the correct screen.
So it's completely random what screen pinballx picks for backglass and dmd. The order in windows stays fixed so in Windows, it does not change on reboots, only in pinballX.
Also, in Visual Pinball, it always is correct. I configured B2S to use screen 1 for backglass, with an x offset of 1920 . So it is not configured to use screen 2 or 3.
However, pinballX does not allow me to select screen 1 for both playfield and backglass (with an x or y-offset) because than the backglass is not displayed at all on any screen.