I can not edit the “Cancel Return URL from PayPal”, its always reset to the default value, after saving settings.

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'] );