I have 2 servers setup on docker, reverse proxy & web server. This error started happening for all my websites all of a sudden after 1 year. When setting up earlier, I generated a self signed certificate on the web server.
So, I had to generate the SSL certificate again and it started working...
openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout ssl.key -out ssl.crt