ok, found the solution.
Change the query in
PermissionControl.loadGroupRoles = SELECT r.name, '1234567890123456789012345678901234567890' as role_value from jforum_roles r where 1=0 \
UNION ALL SELECT r.name, '0' AS role_value FROM jforum_roles r WHERE r.group_id IN (#IN#) \
UNION ALL \
SELECT r.name, rv.role_value \
FROM jforum_roles r, jforum_role_values rv \
WHERE r.role_id = rv.role_id \
AND r.group_id IN (#IN#) \
ORDER BY name
This solves the problem in MySQL 4.0
[originally posted on jforum.net by enricod]