I know this post is 9 months old but I was having the same problem. This post did not help me but I figured out my particular problem. So this is for those where my solution to my problem may help.
For me, I had owned a full version of Melodyne Essentials taht was on my old computer. But then I bought a new computer and also Melodyne Studio. My VST files are on a separate drive from my system drive. That drive got reconnected to the new computer. The VST scan kept failing because there was an old Melodyne Essentials DLL file in a folder named Melodyne. The Melodyne Studio DLL installed in a folder named Celemony Melodyne. The new one succeeded. The old one failed because it was not registered nor installed but the error message did not help me find this folder with the bad DLL. I just got so frustrated that I finally said "What if....?" I deleted the Melodyne folder with the old DLL and all was well. Hope this helps someone else in my position.