Postgres returns error when a feed is open:
ERROR: invalid reference to FROM-clause entry for table "p" at character 285
HINT: There is an entry for table "p", but it cannot be referenced from this part of the query.
STATEMENT: SELECT f.forum_id, f.forum_name, p.post_id, p.topic_id, p.post_time, p.post_subject, p.post_text, p.bbcode_bitfield,
p.bbcode_uid, p.enable_bbcode, p.enable_smilies, p.enable_magic_url, u.username, u.user_id FROM phpbb_posts p, phpbb_users u
LEFT JOIN phpbb_forums f ON (f.forum_id = p.forum_id) WHERE p.topic_id IN (3666, 3832, 3830, 1655, 3587, 3828, 2876, 3815,
3829, 367, 3761, 3827, 3826, 3175, 3451, 3632, 3822, 3823, 3597, 3824)
AND (p.post_approved = 1
OR p.forum_id IN (2, 3, 4, 6, 7, 8, 10, 11, 14, 15, 16, 17, 18, 23, 24))
AND p.post_time >= 1266952354
AND u.user_id = p.poster_id ORDER BY p.post_time DESC
LIMIT 20 OFFSET 0

Switching the two tables in the FROM part did the trick:

— feed-old.php 2010-03-03 23:49:23.637834100 +0100
+++ feed.php 2010-03-03 23:49:30.523762619 +0100
@@ -783,8 +783,8 @@
'p.post_id, p.topic_id, p.post_time, p.post_subject, p.post_text, p.bbcode_bitfield, p.bbcode_uid, p.enable_bbcode, p.enable_smilies, p.enable_magic_url, ' .
'u.username, u.user_id',
'FROM' => array(

• POSTS_TABLE => 'p',
  USERS_TABLE => 'u',
  + POSTS_TABLE => 'p',
  ),
  'LEFT_JOIN' => array(
  array(