I found this error in <FO>/cache/log.php, I would like to know how to fix this, although I haven't got any errors when using FO but this log keeps growing very fast to log this error repeatedly.
#1 ERROR: Error: String functions are overloaded by mbstring in '/srv/www/htdocs/feng/library/utf8/utf8.php' on line 39 (error code: 256)