My custom reports work fine, but mail_report fails
What doesn't help, is that the code for Report.php:402

https://github.com/civicrm/civicrm-core/blob/master/CRM/Report/Utils/Report.php#L402

requires

if (strstr(CRM_Utils_Array::value('name', $templateInfo), '_Form')) {


and otherwise

  $is_error = 1;
  // ...
  $messages[] = 'Did not find valid instance to execute';


The 'name' reported for my custom report is more like

com.foo.bar


.. is that a bug, or some requirement I missed ?