hozansahin Hello, you are correct, I was able to reproduce the issues!
The Nginx Configuration editor has a fallback mechanism and if there are syntax errors in the file or nginx status is failed after saving, then it will restore previous configuration - that is what it is doing in your case!
Login to that user via command: opencli user-login USERNAME
and the do "nginx -t" - it will show what exactly is causing the problem. If it says that there are problems with the default file, then run:
echo "server { listen 80 default_server; listen [::]:80 default_server; server_name _; deny all; return 444; }" > /etc/nginx/sites-available/default && service nginx restart
This will start nginx, and afterwards what you enter in the configuration will be saved correctly!