The Error 0xC004C020 means the Activation key has exceeded its limit as described in the Microsoft Knowledge Base article below. As you have activated the server once, it is expected behavior.
KB 942969 http://support.microsoft.com/kb/942969/en-us
In this case, you need to call Activation Center to activate the server.
Please perform the following the steps:
1, Run the command "slui 4" from the Elevated Command Prompt
2, Please choose the nearest location where you live, click next
3, Call the Activation Department with the telephone number provided by the last step
4, Provided them with the Installation ID and they generate a corresponding Confirmation ID.
5, Enter the Confirmation ID
You will be able to activate the Windows Server 2008 Successfully.
For more information about the Windows Activation, please refer to:
933177 You may be unable to activate Windows Vista over the Internet by using the Windows Activation Wizard
http://support.microsoft.com/default.aspx?scid=kb;EN-US;933177
[original article https://social.technet.microsoft.com/Forums/windowsserver/en-US/7df79f6c-3d89-42ce-bebd-833dce637221/windows-2008-activation-problem]