Ok, so this turned out not to be a missing field problem. The problem is if you have a forum where users can view, but then subforums that they don't have access to. There is a slight change that needs to be applied to scripts/postlist.inc.php to fix this.
Line 1186, basically just about at the end, looks like this:
} // end foreach
return $inlist;
} // end get_subforums
Just change that, to this:
} // end foreach
if (!$inlist) $inlist = "''";
return $inlist;
} // end get_subforums