After first install to use with sql 2008 express r2 (mysql work fine)
It will show APPLICATION WARNING #user_get_field() for NO_USER:
on login page and cannot use default username/ password to login

after search in halfday i found this solution is fix

go to library\adodb\adodb.inc.php in mantis folder and edit this line
define('ADODB_FETCH_ASSOC',2);
to
define('ADODB_FETCH_ASSOC',0);

for me this line is 110

now warning is gone and can login normally.