Cloudflare Blocking ConvertKit Email Broadcast?

I’m setting up a ConvertKit broadcast email. When I try to preview it in a browser, though, I see “Attention Required! Cloudflare” in the title bar with no content in the window itself. In a “view source” I see “Sorry, you have been blocked” message.

The email is HTML-based and I have several references to my fully-qualified domain name. I thought that perhaps Cloudflare was detecting a non-human attempt to access the site and blocked it. But even if I enable “development” mode and purge the cache, I see the same problem.

Anyone know what’s going on?

The ‘Attention Required! Cloudflare’ suggests you are getting challenged by one our our security features.

You can look under the ‘Firewall’ tab of our dashboard and look under the ‘Firewall Events’ to look up what security feature is challenging you there and then tweak your rules to allow this request - https://support.cloudflare.com/hc/en-us/articles/360024520152-Understanding-Cloudflare-Firewall-Analytics