Several customers have recently had problems with uninstalling old versions of the SketchUp Octane plugin when new versions are installed. This is caused by an incompatibility between the old and new installer builds, which we can't fix just by changing the new installers (and we can't change the old ones because our time machine is in for repairs). So what I've done instead is create a small utility you can run if you're having uninstall problems:
https://render.otoy.com/downloads/ba/63/ca/8f/UnplugPlugins.zip
This will simply remove all existing SketchUp Octane plugins from your system, along with their uninstall database entries, leaving you with a clean slate so you can install a new plugin without any problems with existing ones. This will only touch the Octane plugins, it will not change anything else in your SketchUp installation. You will need to run it as Administrator; it can be run either from Windows Explorer (right click, choose "Run as administrator"), or from a command line window that has been opened in administrator mode.