Unfortunately I was not able to dig what is causing this, this is just taken from demo server logs.
On POSTrequest on server_privileges.php, I got following error:
PHP Fatal error: Call to a member function isSuccess() on a non-object in server_privileges.php on line 1439#012
PHP error should not occur anymore
QA_3_4 has
if ($message instanceof PMA_Message) {
PMA_ajaxResponse($message, $message->isSuccess(), $extra_data);
}
opposed to
PMA_ajaxResponse($message, $message->isSuccess(), $extra_data);
which was there at the time of the bug report.