[PetrusV]

The following worked flawlessly after I added the column 'test' in calibre. Also note its case sensitive. It will not work if you add 'Test' column in calibre and then use 'test' on the config:
$config['cops_calibre_custom_column'] = array ("test");

If you have more than one custom column then add them like this:
$config['cops_calibre_custom_column'] = array ("test1", "test2", "test3");