Massimo,
This usually occurs when a database doesn't get setup 100% correctly or mostly when you transfer from Wall.fm to a host and use Oxwall. Here's the solution:
"Such problem appears after migration from wall.fm to oxwall using "Data importer" plugin. To fix it, just run this query in your phpMyAdmin:
INSERT INTO `ow_base_menu_item` ( `prefix`, `key`, `documentKey`, `type`, `order`, `routePath`, `externalUrl`, `newWindow`, `visibleFor`) VALUES ( 'admin', 'sidebar_menu_plugins_add', '', 'admin_plugins', 3, 'admin_plugins_add', NULL, 0, 2), ( 'admin', 'sidebar_menu_themes_add', '', 'admin_appearance', 3, 'admin_themes_add_new', NULL, 0, 3);
"
And you can view the solution's original topic from here: http://www.oxwall.org/forum/topic/3698
~Jake