Iconizer not compatible with Drupal CSS compressor

I found the issue with this behavior. I'll post an update soon for D7.
In the meantime you can edit the module and replace "_iconizer_add_css()" function in iconizer.php with the following code:

function _iconizer_add_css() {
  $options = array('type' => 'file', 'media' => 'screen', 'preprocess' => TRUE, 'every_page' => TRUE);
  
  // Adding Admin icons
  if (variable_get('iconizer_admin_icons', 1) == 1 && arg(0) == 'admin') {
    drupal_add_css(_iconizer_get_admin_css(), $options);
  }
  
  // Adding files icons
  if (variable_get('iconizer_files_icons', 0) == 1) {
    if (iconizer_is_page_enabled(variable_get('iconizer_file_icons_visibility_pages', ''), variable_get('iconizer_file_icons_visibility', 0))) {
      drupal_add_css(_iconizer_get_files_css(), $options);
      
      // Adding File-icons overrides
      drupal_add_css(drupal_get_path('module', 'iconizer') . '/files_icons-override.css', $options);
    }
  }
  
  // Adding protocol icons
  if (variable_get('iconizer_proto_icons', 0) == 1) {
    drupal_add_css(_iconizer_get_proto_css(), $options);
  }  
}