I’ve been checking the domain you sent me and can’t see any problem with your certs or the way they are offered “now” by your web server.
As I said, I’ve checked the certificates issued to your domain by Let’s Encrypt during last year and all seems good.
That message appears to all the clients trying to access the web site or is only you? If you are using and antivirus, double check that it is working fine because some of those antivirus work as a MITM (Main In The Middle) and maybe that could be the problem.
If you see that message again, the browser should give a link to show the certificate, check it to know what is the domain or domains used in that certificate, that could give a clue to know what’s going on.
Hi sahsanu,
thanks for your answer. The domain went completely offline while Hestia had a green tick at SSL. It was offline for everyone, also the uptime checkers. Perhaps it had to do with Cloudflare, so I changed the nameservers to Hetzner’s, where Hestia runs. I’ll see if that helps. The problem was always solved, when I issued a new certificate via Hestia.