This may happen when the paypal account details have not been updated or deactivated. A member had to put in the additional details such as address and reinstate the paypal account.
I’ve had a problem with being scammed, to all appearances via PayPal, so I don’t want to use it any more. So is there a way I can pre-pay sessions by bankcard instead? If not, maybe this should be an option, given that some other members don’t trust PayPal either?