Express Checkout Controllermodule Not Found

Using PayPal sandbox settings, PayPal Express Checkout allows me to add a product to cart and log onto PayPal with my test account, but upon clicking "continue" button I get:
Fatal error: Class 'Controllermodule' not found in /cart/system/engine/loader.php on line 28

Using PayPal Standard, on the other hand, allows me to complete the transaction.

My line 28 in loader.php is "$controller = new $class($this->registry);"