Hi,
Yes, I ran into the exactly same Problem. I then checked MAC-Adresses, and I saw, that the MAC Adress I wanted to add was already about 200 times as an object in SCCM. That was, because the SCCM Boot CD was in the DVD-Rom, but the MAC Address wasn't in SCCM. So now, evertime the Computer made a restart, and booted from the SCCM CD, it connects to the SCCM Server and the SCCM Server doesn't know the Object. So it always creates a new Objekct named "Unkown", and with the MAC Address of my Server. I then shut the Server down, deleted all "Unknown" Object, and after that, I was able to add this MAC Address. Hope this helps