after upgrade, installer shows white screen
http://domain/administrator/index.php?option=com_akeeba
this entry in administrator/error_log
[22-Dec-2015 12:36:29 countryxxx/locationxxx] PHP Fatal error: Call to undefined function Akeeba\Engine\Util\mb_convert_case() in /home/user/public_html/administrator/components/com_akeeba/engine/Util/Complexify.php on line 280