Regional settings

I cannot seem to change short date format under regional settings for NT user. Can change under local admin just fine. User has no special profile defined, standard settings.

When trying to change setting under regional settings user sign-on toMM/dd/yy recieve error: one or more of the characters you typed in this field is invalid. Try typing differnet characters.

This occurs after manually trying to enter this or using the predefined setting in the drop box. Under admin local all iscorrect and this is set for default