yo,

die GROUP BY KLausel fehlt.

SELECT B.id, B.username AS Nickname, B.geschlecht AS Geschlecht, B.age, B.isonline, count(*) AS Kommentare
FROM präfix_tabelle2 AS A
INNER JOIN präfix_tabelle2 AS B ON A.username=B.username
GROUP BY B.id, B.username AS Nickname, B.geschlecht AS Geschlecht, B.age, B.isonline
ORDER BY Kommentare desc LIMIT 50;

Ilja