Error reference number: 1020 happens when you are using Cloudflare to protect your website against online attacks and to speed up your website and some how Cloudflare firewall rules gets triggered.
This error is not really common and may happen rarely when there is actually an attack or there is some false positive. Either you (the site owner) or your customers might be facing this error.
Also read: Easily Remove WordPress Malware
What is the reason for "error reference number: 1020"
Cloudflare firewall is a set of rules that protect a web application against online attacks like XSS, SQL Injection and more.
This issue can happen if user is trying to submit a data on web application which triggered a rule, or it can also happen if user is trying to access a protected endpoint.
One of the easiest way to fix the error would be to disable Cloudflare Proxy
You can see two arrows, on the upper arrow it says "Proxied" and the other one says "DNS Only". If you turn your record to "DNS Only" this will fix the error, but it means you are prune to attacks and your server IP is exposed.
The second way is to check your Cloudflare Firewall rules logs. As you can see in the image below that user trying to access /www/wp-config.php was blocked and you can see that Rule ID 9ce4e284ff2a486aaa37d642bff5a079 was triggered and user access was blocked.
However in this case I believe this was a malicious user trying to access the sensitive file that should not be accessed.
You can also disable this rule:
If you exclude the rule, you are also exposing your site to certain type of attacks. So make sure if the issue was actually a problem or was it a false positive.
What can you do as a site visitor?
Well, If you have received "error reference number: 1020" as site visitor, then all you can do is contact the site owner with Ray ID that you will be given by Cloudflare.
You can email this Ray ID to the person who owns the site, they can then track down and white list your IP incase Cloudflare is wrongly blocking your access
Although the error called "error reference number: 1020" is not very common, but when this happens it really annoys your end user.
It is best to check your Cloudflare Firewall logs and debug this issue, and if you are site visitor then contact the site administrator to get this issue resolved.