[mariosipad]

@PeterT

I use Apache not NGINX so I have no clue how to fix the rewrite rules (in Apache in var/www/html/cops/.htaccess).

But .... you do not need $config['cops_use_url_rewriting'] = '1'; if you put your calibre library just below the root of cops (in: /var/www/html/cops/calibre-library ) like this in config_local.php:
$config['calibre_directory'] = './calibre-library/';

A full path won't work.

This either real (put the file in the library in that directory) or through a symlink.

Ok, this is not the recommended way to do it, but if you do not expose cops to the WAN it should pose no real security risk.

Hope this helps until you can tweak the rewrite rules.