504 Installation failed plugin/theme – nginx proxy cache

[ Moderator note: moved to Fixing WordPress. ]

Hi guys, I hope somebody knows a solution for this problem:

I setup a WordPress which it uses Plesk: ‘Nginx proxy cache -> apache -> WordPress -> Redis -> MySQL’

Everything looks good but when I’m trying to install a plugin ( plugins > add new ) or theme on WordPress I’m getting Nginx timeout error* I don’t have an idea why Nginx give that error because I’d thought it would only handle incoming requests.

If you have an idea for a solution or you know a better place for this question please give me a feedback.

My Nginx Configuration: https://gist.github.com/anonymous/2a9e2a44c28560a5647ee1ad515051d2

Nginx Error:
Installation failed: 504 Gateway Time-out 504 Gateway Time-out nginx padding to disable MSIE and Chrome friendly error page -> < ! – a padding to disable MSIE and Chrome friendly error page ->

Also, I opened the same topic on other forums too :
> http://community.rtcamp.com/t/504-installation-failed-plugin-theme-nginx-proxy-cache/8145
> https://talk.plesk.com/threads/504-installation-failed-plugin-theme-wp-nginx-proxy-cache.341486/