Capture One Pro 12. I tried uninstalling, rebooting, reinstalling - same.
Should have mentioned first

that I had tried installing the sample plugin from the Capture One SDK. It did nothing, so I removed the plugin. I believe (not 100%) that it was after that where this problem started. The plugin was an empty framework and the install is just to make it available in the plugins folder - removing the folder should be all that's needed to remove the plugin.
At this stage I'm at the mercy of the support team. I do have it still running on my laptop so at least can use that for my tethered shoots (I have one tomorrow!) but don't have a desktop install until this is resolved.