I can equip it and use it but it is invisible. Also I can't place it on a shield rack cause the game doesn't recognize I have one.

the emperor's new shield eh? sounds like a bug to me, have you triend saving and reloading? or reloading a save from before you aquired the shield? Also, what happens if you sell it to a merchant?

Happens to every Ebony Shield I find and I can sell it just fine too. I did find my first one around level 10 or so, think maybe I found it too early?