When customer with available balance (unapplied open payment or credit memo) placed a new order for total less or equal than avaialble balance, an order is processed without prepayment. However when making a new purchase from the online store (or from CCP with configured redirect to the store) a customer is still asked for the payment method. The customer has possibility to choose a redirect payment, a request to create a payment for 0 sum will be done in this case. Most systems do not support requests for 0 amount, the customer will get an error. An error depends on the payments system used, it can be:
Failed to prepare redirect - for Worldpay Payment Gateway Invalid Amount - for Assist
An issue is caused by the bug with id #PBA-61082.
Set an option
CANCEL_FRAUD_CHECK to true in the online store: Home > Products > Online Store > Advanced Settings
And synchronize the store.
With the option set to true the customer will still need to choose a payment method, but no attempts to create a payment for 0 sum (fraud check) will be done.