Troubleshooting a UniqueDomainRequired Error ordering MarketConnect Services

Symptom

After applying the 8.0.0 update, an Oops fatal PHP error can occur when an existing client attempts to purchase a MarketConnect service using a domain which is already in their account.

WHMCS\Exception\Domains\UniqueDomainRequired in /path/to/whmcs/vendor/whmcs/whmcs-foundation/lib/Domains/Domain.php:0

Solution

Please apply the 8.0.1 Maintenance Release via the Utilities > Update WHMCS page.

Hotfix

A hotfix has been created for 8.0.0 only to better improve the domain validation in this situation:

https://whmcs.community/files/file/135-core-15412-improve-domain-validation-for-marketconnect-orders-as-an-existing-client/