So it turns out the location of the munin directory for output was wrong. The docs pointed to /var/www/munin, but the www directory for nginx is in /usr/share/nginx/www, so the munin directory must be located in /usr/share/nginx/www/munin (or a symbolic link can be created) and permissions must be s...

I did manage to resolve the error with export MUNIN_LIBDIR=/usr/share/munin, but no output is being generated in /var/www/munin and all I see in the browser is the "Welcome to nginx!" heading.

I encountered the same error. What did you re-do to eliminate the error condition? BTW, nginx is up (there's a welcome screen), but nothing is being generated to /var/www/munin.