Nginx does not start anymore

Hello,

I am running the latest Plesk on the latest stable Debian. After one of my sites experienced a 504 Nginx Gateway Error I followed the official Plesk resolution advice on the following site:
https://kb.plesk.com/en/120210

I edited the nginx.conf as stated.
At the end it says: Restart 'php-fpm', 'apache' and 'nginx' services.

I restarted php-fpm, worked
restarted apache, worked
tried to restart nginx, took very long and I already knew something did not work out. It did not...

Nginx does not start anymore