2. Camel routes the message to a JBI service-unit, which performs some operations based on the data.
3. The service-unit then uses JBI's DeliveryChannel to send the exchange back to the ReplyTo with the outMessage set with string content.
Can you elaborate more on what JBI SU you are using and if possible attach the configuration to investigate further? It looks like some one in the path is either configuring wrong messge type or something of that sort but need to look more into what is being used and how.