Spring Integration (java dsl) ERR_INVALID_CHUNKED_ENCODING

So i want to provide a http-endpoint in Spring Integration by using java dsl:

        return IntegrationFlows.from(Http.inboundGateway("/my-http-path")
                    .requestMapping(r -> r.methods(HttpMethod.GET))
                    .headerMapper(headerMapper)
                    .errorChannel(DEFAULT_HTTP_ERROR_CHANNEL)
                    .requestPayloadType(String.class)
        )
            .scatterGather(
                    scatterer -> scatterer
                            .applySequence(true)
                            .recipientFlow(firstFlow)
                            .recipientFlow(secondFlow),
                    gatherer -> gatherer
                            .releaseStrategy(group -> group.size() == 2)
                            .outputProcessor(mainFlowProcessor))
            .get();

I remember this code working a few months ago. Testing it today tho, it doesn't seem to work anymore. At the end there should be a JSON returned which looks as follows:

@Builder
public class MyResponse {
  @JsonProperty("firstProp")
  public int[] firstProp;

  @JsonProperty("secondProp")
  public int[] secondProp;
}

Does anybody know, why this doesn't work?