So the MTA server is currently installed into /root/server
nginx can not read files inside /root, so move the MTA server to another directory, like /srv
mv /root/server /srv
Then edit /etc/nginx/sites-available/default
and change:
root /server/mods/deathmatch/resource-cache/http-client-files;
to
root /srv/server/mods/deathmatch/resource-cache/http-client-files;
and restart nginx
service nginx restart