From the COPS
wiki-link:
If you use apache as webserver on your Raspberry just change a setting in config_local.php
Code:
/*
* use URL rewriting for downloading of ebook in HTML catalog
* See Github wiki for more information
* 1 : enable
* 0 : disable
*/
$config['cops_use_url_rewriting'] = "0";
to
Code:
$config['cops_use_url_rewriting'] = "1";
That should fix it.