This article covers methods to resolve Cloudflare ERR_SSL_PROTOCOL_ERROR. Basically, this error happens as a result of a number of reasons that include wrong DNS settings, SSL mode, TLS version or issues with SSL certificate.
To fix this error,
1. If you are not the site owner, contact the site owner and let them know you are having issues accessing their site.
2. The domain name has not fully been set to use Cloudflare yet. Check to see if there are non-Cloudflare nameservers that are conflicting with the assigned Cloudflare name servers.
3. You are signed up for Cloudflare, but you have set a DNS record to :grey:. If you have a subdomain or hostname and that serves HTTP/HTTPS traffic, we would advise that you :orange: this DNS record to take advantage of Cloudflare’s security and performance features. See What subdomains are appropriate for :orange:/:grey: clouds? 69
4. The Free Universal SSL certificate hasn’t yet been deployed, :grey: (deactivate) Cloudflare so that your website can continue to use your origin’s SSL certificate. Wait 24 hours and :orange: (activate) Cloudflare again to see if your SSL certificate has been successfully deployed.
5. You have some conflict with TLS 1.3, disable it and see if you still encounter the error.
6. You have selected Full SSL (strict) under your SSL/TLS app, but your origin does not have a valid certificate (i.e., it is expired, self-signed, or not issued by a trusted CA). Temporarily select another SSL option until you have a valid origin certificate in place.
7. Try to look at developer web console (firefox Ctrl+Shift+K) - anything interesting under Security tab.
8. Also, Try to run the command curl -vk on the affected domain.