Basic Security Challenge custom page no longer working

I have had a Custom Page for my Cloudflare Basic Security Challenge for over 4 years. It worked fine. Recently, it no longer works. Although I have the ::CAPTCHA_BOX:: in the HTML file, the box does not show up. The users who have been challenged have no way of proving that they are not a Bot.

When I try to preview the Custom Page in the Cloudflare console, it gives me an error:

Custom error preview failed. Status code 500 received while fetching template from https://www.hockeydb.com/challenge.html

The page is a HTML page, and there is no reason it should be getting a 500 error. In fact, since my challenge page is rarely requested, I can see the request by what is likely Cloudflare in my server logs, and my logs say that the page was returned successfully (200 error).

Thanks if anyone can help!

To confirm I’m following, you had this exact page for the last years, and suddenly it stopped showing the CAPTCHA? Can you gif/screenshot what the users see while facing the captcha? A log of network errors and the source of the captcha page presented to the users can be helpful.

Yes, the exact same page for the past 4 years, no changes whatsoever, and all of a sudden the Custom page stopped working.

Here is what a user sent me, and I verified this myself by challenging my home IP:

Cloudflare will not let me set a Custom page anymore - I can’t preview it, and it tells me “your custom page must be more than 100 characters”.

You can see the HTML page I am trying to set here:

https://www.hockeydb.com/challenge.html

The issue seems to be that Cloudflare thinks it can’t access this page from my server. But here is what seems to be Cloudflare’s request from my server logs:

www.hockeydb.com 172.70.6.30 - - [24/Nov/2021:07:55:54 -0500] “GET /challenge.html HTTP/1.1” 200 4835 “-” “Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10; rv:60.0) Gecko/20100101 Firefox/60.0 Collapsify/0.6.0 node/v10.16.0”

The 200 is the success HTML code, and the 4835 is the number of bytes returned.