PDOException: General error: 1111 Invalid use of group function

For a rather complex dynamic query that my module generates, the above exception is always thrown: "PDOException: SQLSTATE[HY000]: General error: 1111 Invalid use of group function".
The produced SQL code is the following:

SELECT t.item_id AS item_id, SUM(t.score) AS score
  FROM (
    SELECT t.item_id AS item_id, SUM(t.score) AS score, :word1 AS word
      FROM (
        SELECT t.item_id AS item_id, SUM(t.score) AS score, :word1 AS word
          FROM (
            SELECT t.*
              FROM {search_api_db_2_1_title} t
              WHERE ( (word = :db_condition_placeholder_0) OR (word = :db_condition_placeholder_1) )
            UNION ALL
            SELECT t.*
              FROM {search_api_db_2_1_body_value} t
              WHERE ( (word = :db_condition_placeholder_0) OR (word = :db_condition_placeholder_1) )
            ) t
          GROUP BY t.item_id
          HAVING (COUNT(DISTINCT t.word) >= :subs2)
        UNION ALL
        SELECT t.item_id AS item_id, SUM(t.score) AS score, :word2 AS word
          FROM (
            SELECT t.*
              FROM {search_api_db_2_1_title} t
              WHERE ( (word = :db_condition_placeholder_2) OR (word = :db_condition_placeholder_3) )
            UNION ALL
            SELECT t.*
              FROM {search_api_db_2_1_body_value} t
              WHERE ( (word = :db_condition_placeholder_2) OR (word = :db_condition_placeholder_3) )
            ) t
          GROUP BY t.item_id
          HAVING  (COUNT(DISTINCT t.word) >= :subs2)
        ) t
      GROUP BY t.item_id
    ) t
  WHERE (t.item_id NOT IN (
    SELECT t.item_id AS item_id
      FROM (
        SELECT t.item_id AS item_id
          FROM {search_api_db_2_1_title} t
          WHERE  (word = :db_condition_placeholder_0)
        UNION ALL
        SELECT t.item_id AS item_id
          FROM {search_api_db_2_1_body_value} t
          WHERE  (word = :db_condition_placeholder_0)
        ) t
      HAVING  (COUNT(DISTINCT t.word) >= :subs1)
    ))
  GROUP BY t.item_id
  HAVING  (COUNT(DISTINCT t.word) >= :subs1)
  ORDER BY score DESC

However, when I enter the exact same code (just with placeholders replaced) into phpMyAdmin, the database has no problems with it. Therefore I assume that this is a restriction imposed directly by PDO – is that reasonable?

In any case, I'm at a complete loss as to why this exception is thrown. As far as I know, the query looks completely correct (and, as said, the MySQL server seems to agree). Following some related solutions on the internet (although they didn't exactly apply to this problem) I even tried to move all COUNT(DISTINCT word)s from the HAVING clauses directly to the SELECT (as a selected expression), and then filter on HAVING (num >= :subs2), but the problem remained.