CloudFlare SSL / HTTPS issues with landing page custom domain
If you're using CloudFlare, having CloudFlare's orange cloud proxy enabled may cause issues with ConvertFlow's SSL certificate generation, depending on your CloudFlare configuration's SSL mode.
If you're using "Strict" or "Full", try disabling the orange cloud for the CNAME. If not, you should be able to keep the orange cloud enabled.
After your ConvertFlow landing page is showing on your domain with HTTPS, you can enable the orange cloud again for added network protection.