There is a line in filter_api.php that reads:
$query = "SELECT DISTINCT $t_bug_table.id

About 10 lines below, the results are accessed below with
$t_id_array[] = db_prepare_int ( $row['id'] );

However, some databases (e.g. SQLite), when selecting in this fashion, will include the table name as part of the key (that is, 'mantis_bug_table.id'). Therefore, I suggest, as a simple fix, to replace the $query line with:
$query = "SELECT DISTINCT $t_bug_table.id AS id

Csaba Gabor from Vienna