I started having the exact problems above in Cubase Pro 10.5 after Melodyne WAS working fine. I finally realized that I had inadvertently applied the extension to several clips beyond the ones I was intending to work on. I’ve always used Melodyne on a collection of the myriad clips that result after comping, but I’ve taken to bouncing these together into a whole verse or chorus and then applying the Melodyne extension. This helps me keep better track of which part of the piece I’m working on and doesn’t seem to be too much of a load for the computer.