You need to derive from SerializedMonoBehaviour, not SerializedBehaviour. There's a serialized version of each Unity class you normally derive from, and SerializedBehaviour is the serialized version of UnityEngine.Behaviour, not UnityEngine.MonoBehaviour. And UnityEngine.Behaviour, of course, does not support coroutines.
oooh sorry, my bad :)
Tor Esa Vestergaard
That's perfectly alright, it's an understandable mistake :)