Invalid SSL certificate on main domain

Often there’s an error on the atheistrepublic.com domain about invalid SSL certificate. It causes the website to go down temporarily, but the forum subdomain remains functional. I’m just wondering if it’s a misconfiguration on the server setup or the firewall needs to possibly be reconfigured (Cloudflare).

Some clarification on that would be appreciated and I’m willing to help if I can to resolve this issue.