I've tried several things. Since the help file mentions that PlanChecker.js was designed to run on a machine without ACP installed, I moved it to a clean machine with ThinScript.dll registered and ASCOM installed, but otherwise similarly to my laptop, with up-to-date Windows 10.

Exactly the same error, so why can't Cscript running PlanChecker.js create object ACP.Plan? Is there something else from ACP that is needed? Surely not a running ACP? My telescopes are overseas connected to other versions of ACP.