Delete and Re-install the Nginx in the Odoo server
Sometimes, due to some unforeseen circumstances, there could be issues with the Https certificate in the Odoo installation server. The errors may be varies and the quick fix is just to remove the nginx and install it again in a clean way.
sudo apt-get remove nginx nginx-common sudo apt-get purge nginx nginx-common
After that, just re-install the nginx and the HTTPS (eg, let's encrypt).