Web server is down Error code 521

After adding a product, the site constantly gives the following error.

Web server is down Error code 521
Visit cloudflare.com for more information.
You can also see the attached error picture.

We have different sites on the server, we do not get this error on them, we only get this error on the site with CS-CART installed.
We have been experiencing this error since we installed CS-CART.

Do you have a suggestion?
I am requesting your support.

There were some recent posts with similar issue. I believe the fix was to disable cdn or clear the cache at cdn.?

The problem is not cs-cart related or server related.
I solved the problem as below.
I entered https://dash.cloudflare.com/.
On the SSL/TLS page.
Create a Configuration Rule to customize these settings by hostname. (I entered.)
I created Create new Configuration Rule.
Why did I stumble upon some features, I turned them on and then turned them off.
and finally i solved the problem.

If everyone who has a problem like me does what I said above, I think the problem will be fixed.