Here's how to change your log levels as required.
How to enable/disable Plesk debug mode
panel.log can be found at /usr/local/psa/admin/logs/panel.log
Source: Plesk for Linux services logs and configuration files
This actually isn't correct.
You need to add "nginxClientMaxBodySize" without a value to the panel.ini to make it work.
[webserver]
nginxClientMaxBodySize =
Without a value you now have the freedom to set ClientMaxBodySize for each domain in the Plesk panel.
This is also stated here: Invalid...
I have the same situation, I'm using nginx (non proxy mode) with Plesk Onyx and looking for a way to use the web application firewall.
I tried the suggested test at http://wiki.atomicorp.com/wiki/index.php/Atomic_ModSecurity_Rules#Step_10:_Test_your_web_server and I get a 404 error, which means...