The "500 Internal Server Error" is a tricky one. It could be a problem with ngnix or with COPS. And that does not help you much.
First start with reinstalling COPS with the latest version from Github:
https://github.com/seblucas/cops or
Code:
git clone https://github.com/seblucas/cops
if you know what you are doing.
Then (in the COPS directory) copy
config_default.php to
config_local.php and link to your calibre directory with the settng
$config['calibre_directory'] = './'; in config_local.php.
Any errors when you connect to COPS with
http://<IP-RaspberryPi>/cops/checkconfig.php ?
The calibre directory should have the accessibility rights 755 (chmod -R 755 calibre_directory). Also try to (re)name the directory without "spaces".
On my Pi (with Apache) it works.