All I can tell you about this is that in order to proceed, the installer needs to get a path from the registry, which is written by Rhino's installer. This path is expected to be found here:
  • HKEY_LOCAL_MACHINE\SOFTWARE\McNeel\Rhinoceros\5.0x64\Install\InstallPath
I suppose that failures to find this might include partial failure of the Rhino installer, removal of the value (or one of its parent keys) by a registry cleaner program, and lack of permissions to read from the registry when running the plugin installer.

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