Dave,
I’ve run into this error a number of times lately — Cloudflare will return a 520 error on /wp-admin/index.php
. Page rules at Cloudflare can make the rest of the back-end accessible, but that particular URL stubbornly returned the 520 error in spite of them.
I’m only just learning that your plugin (which I love) appears to replicate functionality that Cloudflare already provides, so perhaps that’s the conflict.
In any case, in my experience, this little bugger has cropped up on servers with and without HHVM, so I don’t think that’s the issue.