data in the table (piwik_log_visit.referer_keyword) are inserted incorrectly.
Can you post the original referrer using your web server's access log?
To decode some referrers, Piwik needs the iconv/mbstring extensions to convert to UTF-8. Can you confirm you have these PHP extensions?
Please let us know a bit more info as vipsoft requested so we can help and fix the problem.