526 error after enabling load balancing on existing site

I’ve had a site proxied through cloudflare for some time. We use Full Strict SSL validation. The site has been working no problem until today when I activated a load balancer using the same subdomain. One of the origin servers points at my server hosting the site. I use Nginx on the server for reverse proxying. Port 443 is open. When I go to the load balancer domain it says that there is an invalid certificate and throws a 526 error.

I don’t see how the certificate could be invalid since I downloaded it from Cloudflare under the origin server tab and the site has been working till today.

Is there something I’m missing with configuring a load balancer through Cloudflare??