in the security assist wizard, in the validation settings for the password field, the default is to set the Min Special and Max Special to 0.
this is causing the "@" symbol to fail validation. by setting the Min Special and Mac Special to 0 it will not allow any special characters, it will only allow letters and numbers. you need to change the Max Special to at least 1 (set it bigger if you want to allow more than one) in order to allow a special character in the password.