Alright, I guess all of you guys get this error at least once in your life just like me the other days. I was searching around the Internet and found a lot of how-to guides but It did not work for me. Finally, I found out what caused the error of my WordPress instance:
I set the Nginx server configuration file path of W3 Total Cache (>>Performance >> General Settings) as /etc/nginx/nginx.conf which is not writable by the www-data user. So, I set the path to /var/www/nginx.conf (default), save the settings and everything is fine now.