Hi Stuti,
In the Web Service Message Monitor in C4C look at the error and get the payload. Do you have an error in the outbound monitor, or do you only see this in your middleware?
In C4C you can download the payload. In the middleware then look at the payload you see there (do you have an option to turn on payload tracing?).
Also check that the external system ID's you setup match.
Shashank GokhaleMarcus Echter - other ideas of what to check?
-ginger