Ciao,
controlla con phpmyadmin o equivalente di avere la tabella tuoprefisso_tags e che ci siano i seguenti campi popolati:
id, parent_id, lft, rgt, level, path, title, alias, note, description, published, checked_out, checked_out_time, access, params, metadesc, metakey, metadata, created_user_id, created_time, created_by_alias, modified_user_id, modified_time, images, urls, hits, language, version, publish_up, publish_down.
Nel caso esegui la seguente query (trovata in rete):
INSERT INTO `tuoprefisso_tags` (`id`, `parent_id`, `lft`, `rgt`, `level`, `path`, `title`, `alias`, `note`, `description`, `published`, `checked_out`, `checked_out_time`, `access`, `params`, `metadesc`, `metakey`, `metadata`, `created_user_id`, `created_time`, `created_by_alias`, `modified_user_id`, `modified_time`, `images`, `urls`, `hits`, `language`, `version`, `publish_up`, `publish_down`) VALUES
(1, 0, 0, 43, 0, '', 'ROOT', 'root', '', '', 1, 0, '0000-00-00 00:00:00', 1, '{}', '', '', '', 0, '2011-01-01 00:00:01', '', 0, '0000-00-00 00:00:00', '', '', 0, '*', 1, '0000-00-00 00:00:00', '0000-00-00 00:00:00');
Rimpiazza tuoprefisso con il tuo prefisso.
Prima fai un backup.