error message when opeining forum start page

Hi !
I get this error message when I call http://website.com/forum :

    * user warning: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LEFT JOIN vm_localizernode loc ON loc.nid=n.nid LEFT JOIN(SELECT -100 AS weight,' at line 1 query: SELECT tid from vm_forum group by tid LEFT JOIN vm_localizernode loc ON loc.nid=n.nid LEFT JOIN(SELECT -100 AS weight, CAST('de' AS CHAR(10)) AS language UNION SELECT 1 AS weight, CAST('en' AS CHAR(10)) AS language) lanw ON lanw.language=loc.language LEFT JOIN (select pid, MIN(weight) AS minweight from vm_localizernode loc2 LEFT JOIN (SELECT -100 AS weight, CAST('de' AS CHAR(10)) AS language UNION SELECT 1 AS weight, CAST('en' AS CHAR(10)) AS language) lanw2 on lanw2.language=loc2.language group by pid) loc1 ON loc.pid=loc1.pid WHERE (lanw.weight=loc1.minweight) in /www/htdocs/vocalmus/includes/database.mysql.inc on line 172.
    * user warning: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LEFT JOIN vm_localizernode loc ON loc.nid=n.nid LEFT JOIN(SELECT -100 AS weight,' at line 1 query: SELECT tid from vm_forum group by tid LEFT JOIN vm_localizernode loc ON loc.nid=n.nid LEFT JOIN(SELECT -100 AS weight, CAST('de' AS CHAR(10)) AS language UNION SELECT 1 AS weight, CAST('en' AS CHAR(10)) AS language) lanw ON lanw.language=loc.language LEFT JOIN (select pid, MIN(weight) AS minweight from vm_localizernode loc2 LEFT JOIN (SELECT -100 AS weight, CAST('de' AS CHAR(10)) AS language UNION SELECT 1 AS weight, CAST('en' AS CHAR(10)) AS language) lanw2 on lanw2.language=loc2.language group by pid) loc1 ON loc.pid=loc1.pid WHERE (lanw.weight=loc1.minweight) LIMIT 0, 25 in /www/htdocs/vocalmus/includes/database.mysql.inc on line 172.

Any idea ?

When I directly call acategory, everything works as expected.

Volker