Cause
This is caused by the quickbooks_sync module not reading the hooks.php file. This is usually caused after an upgrade or migrate from a different WHMCS install.
Resolution
Simply download the attached quickbooks_sync.php file, then rename the current quickbooks_sync.php file in the module folder – and drop this file in. Then reload the admin page. Then delete this file – and move the original file back in. Voila!