JSON Invalid UTF-8 middle byte

I got this exception when in the Java Client Application I was serializing a JSON like this

String json = mapper.writeValueAsString(contentBean);

and on the Server Side I was using Spring Boot as REST Endpoint.
Exception was:

nested exception is com.fasterxml.jackson.databind.JsonMappingException: Invalid UTF-8 start byte 0xaa

My problem was, that I was not setting the correct encoding on the HTTP Client.
This solved my problem:

updateRequest.setHeader("Content-Type", "application/json;charset=UTF-8");
StringEntity entity= new StringEntity(json, "UTF-8");
updateRequest.setEntity(entity);

Android set content type HttpPost