Troubleshooting a UniqueDomainRequired Error ordering MarketConnect Services
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
Please apply the 8.0.1 Maintenance Release via the Utilities > Update WHMCS page.
A hotfix has been created for 8.0.0 only to better improve the domain validation in this situation: