This issue is exist at the version 4.0 and 4.0.1 of the Booking Calendar.
For fixing this issue, please make this small modifications:
1) please open the ../booking/inc/payments/paypal.php file:
2) find this code:
update_bk_option( 'booking_paypal_return_url' , $_POST['paypal_return_url'] );
and replace it to this code:
update_bk_option( 'booking_paypal_return_url' , $_POST['paypal_return_url'] ); update_bk_option( 'booking_paypal_cancel_return_url' , $_POST['paypal_cancel_return_url'] );