HTTP 521 Error but I don't know why

I am trying to create a subdomain for my NodeRED webinterface so i dont have to remember the port.
I have my my subdomain entered in the DNS through a CNAME which points to my domain. The domain points to my proxy which normaly without any subdomains points to my main server. With the Subdomain for NodeRED it also points to my main server but on the Port 1880. For security I have my SSL/TLS set to Full with the NodeRED Webinterface having a certificate issued by Cloudflare.
Now the Problem is, is that ven though the NodeRED Docker container is up and running and the webinterface is accessible through the servers ip and port, when accessing Cloudflare shows me “Web server is down” HTTP ERROR: 521.
I don’t know why this is happening/ wrong since the Subdomain points to the correct server but somehow it still isn’t available.
Can anyone help me there?
Best regards Justricity

A hostname has nothing to do with a port.

That is not a port supported by Cloudflare.

That’s actually insecure and not for security. It needs to be Full Strict, otherwise you have no encryption.

Bottom line, apart from the port, does everything load fine on HTTPS when Cloudflare is paused?

If not, it’s a server issue.

To recap

  1. Switch to Full Strict
  2. Pause Cloudflare
  3. Change to a supported port
  4. Verify that it loads fine on HTTPS
  5. Once all that worked, it will also work on Cloudflare

You generally have a security issue, as you do not only have an insecure legacy encryption mode, but you also chose the wrong certificate.

You need to get a certificate from and change the encryption mode to Full Strict.

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