Wordpress' file manager plugin won't work

After installing and activating the plugin (https://wordpress.org/plugins/wp-file-manager/) I get the following error:
Unable to connect to backend

Here's a screenshot:
https://pasteboard.co/HAkTSvr.png

I've only installed CWP yesterday and that's the first WP instance I've used. I can just say that disabling mod_security didn't help, and I use this plugin in other hosting companies of VPS (with cPanel, Vesta and ISPConfig) with no issue.

Anyone with an idea what's wrong?

PS: I'm using Apache + Nginx + Varnish

Ok, issue fixed, though I can't really explain why.
All I did was following this guide for enabling leverage browser cache on Apache.

How come it influenced the file manager plugin? beats me...

Leverage cache will work fine
probably permission issue or cache issue, by clearing the wordpress cache and fixing the permission from CWP admin will solve it.
always check error log for more details