Solution:
Get rid of all "AS" after "FROM" in "SELECT ..." statements.

diff -rupN mantisbt-1.1.7/core/adodb/drivers/adodb-oci8.inc.php mantisbt-1.1.7_zce/core/adodb/drivers/adodb-oci8.inc.php
--- mantisbt-1.1.7/core/adodb/drivers/adodb-oci8.inc.php 2009-04-20 09:14:00.000000000 +0200
+++ mantisbt-1.1.7_zce/core/adodb/drivers/adodb-oci8.inc.php 2009-05-13 11:24:18.163250000 +0200
@@ -977,6 +987,13 @@ NATSOFT.DOMAIN =
*/
function _query($sql,$inputarr)
{

• // remove 'as' after 'from'
• while ($sql != preg_replace('/(.\bfrom\b.?)\bas\b(.*)/is', '$1 $2', $sql)) {
• $sql = preg_replace('/(.\bfrom\b.?)\bas\b(.*)/is', '$1 $2', $sql);
• }

• if (is_array($sql)) { // is prepared sql
      $stmt = $sql[1];

I think I have seen a similar solution somewhere around (here?). But I can't find it anymore.