ESXi 5.1 AD authentication

I am using windows 2003 active directory.

first I install a esxi 5.0 without vCentor, and config to use AD authentication.

when login in the with the vSphere Client, I check the "use windows session credentials" box, it can login.

now, I install a esxi 5.1, the same config as the esxi 5.0.

but when login in the with the vSphere Client, I check the "use windows session credentials" box, it can not login.

the login format is : "domain-name\user-name"

if I type uncheck the "use windows session credentials" box, and type the use name as "user-name@domain-name", and the user active directory's password, it can login.

it is my problem or the esxi 5.1 problem?