The problem is an unusual using of the DISTINCT command. In the line 164 you will find the line:

$query3 = "SELECT COUNT( DISTINCT ( $g_mantis_bug_table.id ) ) as count " . $t_from_clause . $t_where_clause;

just replace it with this:

$query3 = "SELECT COUNT( $g_mantis_bug_table.id ) as count " . $t_from_clause . $t_where_clause;

That's it! You really don't need to distinct the queryin this case.

yours
Matthias