BuddyPress Activity plugin and 520 error

I am having an issue with the BuddyPress Activity plugin - I keep getting this error when trying to post links from medium.com: 520: Web server is returning an unknown error

There is an unknown connection issue between CloudFlare and the origin web server. As a result, the web page can not be displayed.

I have checked with our web host, who says it's not on their end. I am wondering if it's something in the way BuddyPress Activity processes a link from Cloudflare?