[Admin]

Try this.
In your showthread.php, add the code in red

error_reporting(7);
if ( isset($goto) and ($goto=='lastpost' or $goto=='newpost')) {
  $noheader=1;
}

if ($bbuserinfo[userid]!=0) {

and also this

getforumrules($forum,$getperms);
eval("dooutput(\"".gettemplate("showthread")."\");");

} else {
  show_nopermission();
}

This should work (untested).