After moving the foss.trainingserver to a new location, we found that TLS certificate was expired. I had looked and figured out that though certbot renewed the certificate, it never reloaded the nginx.
Now to make sure that nginx is reloaded next time, we must add the renew hook in the
/etc/letsencrypt/renewal/foss.training.conf under the
renew_hook = service nginx reload
One must remember to update the path based on their DNS. Thank you Saptak for pointing to the expired certificate and mentioning that it is a common pain point for people. I hope that this will be helpful for people in the future.