When AE throws an exception UIMA wraps it in AnalysisEngineProcessException which is handed off to UIMA AS. This exception is subsequently serialized as a java object and sent in a reply to the client. The client deserializes the cargo back to a java Exception and hands it off to the application via a callback. The problem is that the Exception stack trace may include user defined Exception class(es) which are not available to the client thus causing ClassNotFound while deserializing a reply. Modify UIMA AS service side to serialize an exception to a String and send that to the client.