It seems that flexicontent has a problem to update the indexes of the keys 'title' and 'text' to FULLTEXT in the table '#__flexicontent_reviews'.
Because our table '#__flexicontent_reviews' was empty, it worked to manually drop the table and manually reinstall it with the SQL - statement of the newer flexicontent version (/admin/installation/install.mysql.utf8.sql)
Code:
CREATE TABLE IF NOT EXISTS `#__flexicontent_reviews` ( `id` int(11) NOT NULL auto_increment, `content_id` int(11) NOT NULL, `type` varchar(255) NOT NULL DEFAULT 'item', `average_rating` int NOT NULL, `custom_ratings` text NULL, `user_id` int(11) NOT NULL DEFAULT '0', `email` varchar(255) NOT NULL DEFAULT '', `title` varchar(255) NULL, `title_old` varchar(255) NULL, `text` mediumtext NULL, `text_old` mediumtext NULL, `state` tinyint(3) NOT NULL DEFAULT '0', `approved` tinyint(3) NOT NULL DEFAULT '0', `verified` tinyint(3) NOT NULL DEFAULT '0', `useful_yes` int(11) NOT NULL DEFAULT '0', `useful_no` int(11) NOT NULL DEFAULT '0', `submit_date` datetime NOT NULL, `update_date` datetime NOT NULL DEFAULT '1000-01-01 00:00:00', `checked_out` int(11) unsigned NOT NULL default '0', `checked_out_time` datetime NOT NULL default '1000-01-01 00:00:00', `attribs` mediumtext NULL, PRIMARY KEY (`id`), KEY (`content_id`, `user_id`, `type`), KEY (`content_id`, `type`), KEY `user_id` (`user_id`), FULLTEXT KEY `title` (`title`), FULLTEXT KEY `text` (`text`), KEY `state` (`state`), KEY `approved` (`approved`), KEY `verified` (`verified`), KEY `useful_yes` (`useful_yes`), KEY `useful_no` (`useful_no`) ) ENGINE=MyISAM CHARACTER SET `utf8` COLLATE `utf8_general_ci`;