Hi @dung,
Check e.g. OJS 3.1.1 upgrade problem - #37 by akyeame for more on the “Malformed UTF-8 characters” issue.
Unless you manually transcoded your database, make sure that your character set configuration in your old config.inc.php matches the ones in your new config.inc.php – even if these look slightly inconsistent, e.g. utf8 vs. utf-8.
Regards,
Alec Smecher
Public Knowledge Project Team