520 error when FULL (Strict) SSL/TLS

Hi All,

I host a web server at home using IIS 10 and Django.

I selected the Cloudflare origin cert for port 443 in my site setting and when I paused the cloudflare in overview and select FULL (Strict) in SSL/TLS, I can connect my site despite it will pop up warning of security issue in my mobile device. The cert shown is Cloudflare Origin Cert with expiration year of 2036.

However, when I resume cloudflare and select full (Strict) in SSL/TLS, it will show 520 error.

While I select Flexible in SSL/TLS, I also can connect my site and the cert shown is R3 from Let’s encrypt based on universal certifcate set up in Edge Certiciate.

My problem is why it will show 520 error when I resume Cloudflare and select full (Strict) in SSL/TLS.

I don’t think it’s the problem from my IIS server since it run perfectly under Cloudflare origin cert when I pause Cloudflare. Any help will be highly appreciated. Thanks.

Regards
Alan