FYI: the SSL certificate seem to have expired (today) for that website.
Thanks, I noticed. It's caused by having many .tk-domains on the same certificate, and because those domains expired, Letsencrypt can't update the certificate automatically.
Since I only followed online instructions to set this up, I now have to figure out how to fix it.
As long as you know it, for me it's OK. Also I guess that most won't be afraid to go in the browser to "advanced" and "accept the risk and continue".
Good luck with the fix! (Sorry, I can't help with that.)