After some online research I found that the menu root item was missing. I was able to add this in the database using this query:
INSERT INTO `[prefix]_menu` (`id`, `menutype`, `title`, `alias`, `note`, `path`, `link`, `type`, `published`, `parent_id`, `level`, `component_id`, `checked_out`, `checked_out_time`, `browserNav`, `access`, `img`, `template_style_id`, `params`, `lft`, `rgt`, `home`, `language`, `client_id`, `publish_up`, `publish_down`) VALUES
(1, '', 'Menu_Item_Root', 'root', '', '', '', '', 1, 0, 0, 0, NULL, NULL, 0, 0, '', 0, '', 0, 93, 0, '*', 0, NULL, NULL);
Could this have been caused by the import of the menu items? Or mabye by the first attempts that had the but (second languae field)? Anyway, just wanted to let you know and maybe this helps out someone else in the future.