May I ask what troubleshooting steps have you tried already from the article below regarding your error code? 
Furthermore, regarding the Elementor, there is a way to make it work both:
Don’t forget to add your hosting IP address to the Security → WAF → Tools → IP Access Rules with the action “allow” for your Website just in case.
Nevertheless, regarding the SSL certificate, Flexible causes the issues regarding redirection loops, mixed content issues and similar as can be read at the article from below: