After applying the 8.0.0 update, an Oops fatal PHP error can occur when deleting a line item from a quote:

WHMCS\Exception\Authentication\ClientRequired: Forbidden in /path/to/whmcs/includes/clientfunctions.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 resolve this error:

https://whmcs.community/files/file/136-core-15405-error-deleting-quote-line-items/