Sharepoint compatible on Windows 2008R2 Server with Windows 2008 Server

Hello Community

    In a Sharepoint 2010 Server server farm with Windows 2008R2 Server
as the dc and Windows 2008 Server as the member server there seems to be
a problem with “Add” to the admininstratos group on the member server. 
The functional level is Windows 2003 Server.

   I connected to the dc from the member and tried to add the member then
 but the message is that it can’t find that username.

    When Windows 2003 Server this was not a problem. 

    Should I change the dc to be a Windows 2008 Server instead of leaving
it as a Windows 2008R2 Server, is there is some kind of compatibility issue?

    Note: The functional level needs to remain Windows 2003 Server because
of trusts with other servers.

    Thank you
    Shabeaut