To use RoboSpice, the entity (context) using a SpiceManager and the SpiceService must live inside the same process.
RoboSpice acts like a bridge between an Android Service and any other context. And this bridge is Object Oriented, allowing an object based bi-directional communication channel between the context and the service.
That's possible on Android at one single condition : the SpiceService and the entity using the SpiceManager must live in the process. That very special "trick" is a hard coded behavior in Android, called "Local Service Bindbing".
If this condition doesn't hold, it's not possible to bind to a RS service in the correct way. When the SpiceManager will bind to the SpiceService it will then receive a BinderProxy with which it is not possible to share references : all communications will have to be based on Bundles, serializable or parcelable objects. In such condition, RS won't work at all.
I have looked at your project on https://github.com/rovo89/XposedBridge/blob/master/src/de/robv/android/xposed/XposedBridge.java and it looks like you are playing with quite low level things in it, and launching new Zygote processes from there. I assume the processes that you create by forking zygote won't be inside the same process as the SpiceService itself, that's the reason why you get this exception.
I am deeply sorry to say so, but you can't use RoboSpice in this project. If you ever find a workaround, I would be quite interested to hear from you, but I don't think it's gonna be possible, by design of Android itself.