Error messages with domain purchase through Domain Mapping plugin

Hi, our customers are able to successfully purchase domains using the Domain Mapping plugin through our WHMCS system. They receive confirmation emails that the domain has been ordered, etc and the order shows up in our WHMCS system log.

However, the customer receives an error message as shown in the attached screenshot when they order the domain. This is despite everything working.

Do you know why this is happening? It seems the integration between Domain Mapping and WHMCS appears to reject the purchase while still processing it. This is creating some very confused customers. Your help is appreciated. Thank you.