Wersja 2.5.5 rzuca takim błędem (2.5.4 jest ok):

Fatal error: Uncaught Error: Call to a member function get_tax_class() on null in /wp-content/plugins/omnibus/includes/iworks/omnibus/class-iworks-omnibus-integration.php:338
Stack trace: #0 /wp-content/plugins/omnibus/includes/iworks/omnibus/integration/class-iworks-omnibus-integration-woocommerce.php(467): iworks_omnibus_integration->add_message()