Just ran into this issue standing up WSUS on a 2012 R2 server today. Fixed it by using theBackConnectionHostNames registry entry, as described in KB 896861 (http://support.microsoft.com/kb/896861):
- Click Start, click Run, type regedit, and then clickOK.
- In Registry Editor, locate and then click the following registry key:HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
- Right-click MSV1_0, point to New, and then clickMulti-String Value.
- Type BackConnectionHostNames, and then press ENTER.
- Right-click BackConnectionHostNames, and then click Modify.
- In the Value data box, type the host name or the host names for the sites that are on the local computer, and then clickOK.
- Quit Registry Editor, and then restart the IISAdmin service.