NET::ERR_CERT_AUTHORITY_INVALID and you are webmaster?
I am webmaster for https://linuxintro.org. However, when I visited this page in Chrome I always got the message
Today I resolved it after some years. I went to https://letsencrypt.org/getting-started/ followed the instructions and got it working in 11 minutes. And it did not cost anything :)
I am super happy this site exists and makes it so easy for me. I immediately donated to them because they are just so useful :)