Have you disabled Track Members and tested? It really has the author_id in the error in the specific hook but only callable in Status Updates replies.
/* Reply to a status update */
if( \get_called_class() == 'IPS\core\Statuses\Reply' AND !$first AND \IPS\trackmembers\Track::canTrack( \IPS\Member::loggedIn(), 'replyStatus' ) )
{
if( $item->author_id == $item->member_id )
{
$text = \IPS\Member::loggedIn()->language()->get( 'tm_replystatusupdate_self' );
}
...
}
Can you check it? I will release a fix today later for Track Members and I will include this one, if this app is the culprit.