Comment (by Ian Hinder):

According to SSL Checker, the required intermediate certificates have not been installed: https://www.sslshopper.com/ssl-checker.html?hostname=einsteintoolkit.org.

The certificate is not trusted in all web browsers. You may need to install an Intermediate/chain certificate to link it to a trusted root certificate. Learn more about this error. The fastest way to fix this problem is to contact your SSL provider.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2249/new-tls-certificate-for-einsteintoolkitorg