Well, there has to be something different - works on one host/course, doesn't work on other host/course.
The error reported gave a php file and a line of code.
That section of the file reported and line of code:
// Check moduleid exists
if (!$coursemodule = get_coursemodule_from_id(false, $moduleid)) {
throw new backup_task_exception('activity_task_coursemodule_not_found', $moduleid);
}
$classname = 'backup_' . $coursemodule->modname . '_activity_task';
If I read that right, sounds like it can't find backup_certificate_activity_task.class.php.
See if you have in /mod/certificate/backup/moodle2/
backup_certificate_activity_task.class.php
file.
And check permissions/ownerships on all files folders in
/mod/certificate
'spirit of sharing', Ken