Digital Ocean, Node.js, Nginx and Cloudflare (400 badrequest No required SSL certificate was sent)
I also experienced this in one the new websites in my server that uses CloudFlare. The issue was that SSL/TLS > Origin Server > Authenticated Origin Pulls was not activated by default in the new website that shares server with my other sites which already had Authenticated Origin Pulls activated. So I just had to activate that for the new site. In other peoples' cases, that may be checked if there is a need to deactivate it.