520 errors related to Caching...?

General
2

May I ask are all of them on the same server/hosting?

That seems a bit strange. Can the server handle all that requests and the PHP as well with your installed plugins?

I am sorry to hear that, but also afraid there could be few reasons:

  1. Origin host / server does not allow Cloudflare IP address(es) to connect to it (Firewall, etc.) due to the requests
  2. Website / domain has misconfigured SSL settings at origin host/server or even at Cloudflare → under the SSL/TLS tab, or does not have a valid and proper SSL certificate at the origin host / server
  3. WordPress is using WP Super Cache plugin, but maybe you are missing the Page Rules at Cloudflare for it (if needed)

Regarding Cloudflare 520 error, may I suggest you to try looking into below articles to troubleshoot the issue:

May I ask what is your domain name and who is your hosting provider, if so?
Maybe some recent and planned maintenance period at the DC from your hosting provider?

Before moving to Cloudflare, was your Website working over HTTPS connection?
If so, did you had an valid SSL certificate installed at your origin host / server which covers both your naked (root) domain any any other needed sub-domain like www, mail, etc.?

May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?

Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:

Regarding available SSL options at Cloudflare dashboard, check here:

If any other issues appear, follow the needed steps for troubleshooting from article below:

In case you do not have an SSL certificate, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to setup an SSL certificate using Cloudflare CA Origin Certificate:

Last but not least, kindly have a look here for more information regarding correct SSL settings at the SSL/TLS tab on Cloudflare dashboard:

Ports list compatible and supported when using :orange: cloud (proxy mode) on Cloudflare can be found here:

Furthermore, do not skip below step and kindly re-check if Cloudflare is allowed to connect to your origin host to as follows in the below article:

Nevertheless, Cloudflare IP addresses list can be found here:

If so, kindly see the steps from below tutorial how to propperly setup WP Super Cache with Cloudflare for your WordPress Website:

I am happy to hear that you read all of them, but have you tried anything from what is already being suggested in them or from above in this post from me due to the SSL at least?

I am quite surprised about this one, how about below official WordPress repo plugin page and GitHub to submit the issues as well and the Automattic website:

https://github.com/Automattic/wp-super-cache/issues

May I ask have you tried writing a ticket and contacting Cloudflare support due to your account and/or domain issue by two possible ways from below?: