I solve the problem by changing a line in /etc/nginx/nginx.conf file as follows:Virtual Host Configs
include /etc/nginx/sites-enabled/.; // files
It should be:
include /etc/nginx/sites-enabled/*; //directories
I traced this error to Let’s Encrypt on Ubuntu 18.04. Let’s Encrypt on Ubuntu 16 updates the line correctly, but 18.04 did not. The addition (.*) causes all sort of connection issues.
I hope it helps, although it is belated.