If your VSCO Film profiles are stubbornly "not found" in Lightroom Classic CC v8.1

I have solved my problem. I am making this post for other people who have this problem.

I was trying to get VSCO Film working in Lightroom Classic CC 8.1 on Mac.

All the camera-profile settings display an error, "Profile missing".

I already knew that I need to be using a profile for the right kind of camera, and that the camera must be supported. I verified that I was using the Sony presets with my Sony A7R III; I had shot in .raw and the camera was accurately identified as ILCE-7RM3 in the metadata.

Nearby threads​​ noted that the 7.1 upgrade saw a lot of profiles moved, and lots of people had trouble as a result. I made sure that all the profiles are were in folders like:

   /Users/username/Library/Application Support/Adobe/CameraRaw/CameraProfiles/VSCO Film 01 Sony Profiles

I heard rumors there that it might be better if I moved them to CameraProfiles/ but this did not help so I moved them back.

I attempted to import a single profile using the Lightroom camera profile manager UI. This worked, and the profile was no longer listed as Not Found, but I found that the operation had duplicated the profile to CameraProfiles/Imported Profiles/. This process was also tedious. I began to look for an alternative. At that point I noticed that the import also seems to have updated the file CameraProfiles/index.dat. I tried deleting this file and then restarting LightRoom. All profiles were now available.

In summary:

If you are having trouble with VSCO film profiles in Lightroom CC and the profiles are in the right place, delete CameraProfiles/index.dat then restart Lightroom Classic CC.