I had same issue where Video -> General Video Settings -> Open madVR Settings produced the exact same error. I deleted the folder and MC21 reloaded it again and then it worked.
However, it created another problem. My Display Settings automatic change mode Custom feature no longer worked. I no longer switched to 4K based on source like it used to. I could not get it to work no matter what I tried including rebooting. I loaded eventually loaded 21.0.90 from 21.0.83 and it worked again, but the original issue with madVR stopped working. I went ahead and deleted the madVR directory under pluggins and got that working again, but it broke the auto display settings again. WTH?
I'd much rather have the auto display working over madVR adjusting from within MC21. However, both should work. I need help on this one!!!! I've wasted hours trying to get this resolved. Time to have the big dogs take a look...