I see the same error in a client's install, it's filling up the error log pretty quickly. The function in question is:
function do_form_check($check, &$value_hash) {
$doomed = create_form_check(&$value_hash); <<== ( [b]line 1911[/b] )
// print "!$doomed!$check!";
return $doomed == $check ? true : false;
} // end do_form_check
in ubbt.inc.php version 6.5.2