Hi Mark Mason (@docmason)
We have checked your site with the Google Mobile Friendly Tool. But, The Tool is responding with a error message.
Please check with your Google Search Console and fix any configuration issue if there is anything not working.
This Type of Errors are encountered by some Plugins like JetPack. You are not the one who is facing this issues. Please find This WP Forum Discussion.
Please Clear All the Caches from the WordPress Dashboard, Hosting, CDN Like CloudFlare if any.
You can Remove any other Plugins those generate separate Mobile Sites (If have).
You can Remove Unnecessary Link URLs from Google Search Console.
Thanks