Cloudflare Error 526
Cloudflare Error 526: Invalid SSL certificate indicates that your SSL setting in the Cloudflare SSL/TLS app is set to Full SSL (Strict) for that website but Cloudflare is unable to successfully validate the SSL certificate on your origin website / web server.
As you can see from the tip next to the Full (strict) setting, in order for this setting to work as intended, the website needs to have a valid and active SSL certificate from a trusted CA.
Troubleshooting Cloudflare Error 526
Possible solutions to remove this error are applicable on a case-to-case basis:
If you do not have a SSL certificate on your website, then get a SSL certificate issued from a trusted CA and set it up on your website hosting.
If your website's SSL certificate has expired, then get a SSL certificate issued from a trusted CA for your website's domain/sub-domain and replace the old one in your website hosting.
If you are using a CNAME to another hostname in Cloudflare DNS, then ensure that the website has a SSL certificate issued from a trusted CA for your website's CNAME hostname and the target hostname i.e. you cannot use a CNAME target that does not already have https enabled on it.