How to update a DNG Module structure with JSON?

I am following the steps in https://jazz.net/wiki/bin/view/Main/DNGModuleApiScenario2

I have Retrieved the Module structure as JSON in a String.
Then I have PUT the same string back in the same module.
I took care of using the eTag.

When I do the PUT using a RESTClient I get this:
{
    "detailedMessage": "Propagated exception; original message [InternalServerErrorException: Propagated exception; original message [RuntimeException: No supported parser for format: text\/plain]] (More info found at entry [2c67ceec6da1b3cb] in the RM application server log)",
    "errorMessage": "Bad Request",
    "errorStatus": "400"
}

In DNG logs:
2018-03-19 16:40:28,552 [Default Executor-thread-100365 @@ 16:40 <unauthenticated> <Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0@9.26.42.216> /rm/resources/_a7V9wSiTEei0FZBpM_jKog/structure] ERROR ibm.rdm.fronting.server.services.RMRestServiceBase  - CRRRS1007E  A problem occurred in the Requirements Management (RM) server: Entry [ab4899d507e71022]
com.ibm.rdm.fronting.server.exception.InternalServerErrorException: Propagated exception; original message [InternalServerErrorException: Propagated exception; original message [RuntimeException: No supported parser for format: text/plain]]
    at com.ibm.rdm.fronting.server.exception.RRSException.newException(RRSException.java:271)
    at com.ibm.rdm.fronting.server.exception.RRSException.newException(RRSException.java:810)
    at com.ibm.rdm.fronting.server.exception.RRSException.capture(RRSException.java:814)
...
Caused by: com.ibm.rdm.fronting.server.exception.InternalServerErrorException: Propagated exception; original message [RuntimeException: No supported parser for format: text/plain]
    at com.ibm.rdm.fronting.server.exception.RRSException.newException(RRSException.java:271)
    at com.ibm.rdm.fronting.server.exception.RRSException.newInternalServerErrorException(RRSException.java:149)
    at com.ibm.rdm.fronting.server.core.jpi.internal.InternalServiceExchange.checkError(InternalServiceExchange.java:56)
    at com.ibm.rdm.fronting.server.core.jpi.internal.InternalServiceExchange.getResponse(InternalServiceExchange.java:37)
    at com.ibm.rdm.fronting.server.core.spi.BaseServiceRequest.execute(BaseServiceRequest.java:118)
    at com.ibm.rdm.fronting.server.core.spi.BaseServiceRequest.executeWithExchange(BaseServiceRequest.java:128)
    at com.ibm.rdm.fronting.server.services.RMRestService.runInternalMethod(RMRestService.java:469)
    ... 59 more
Caused by: java.lang.RuntimeException: No supported parser for format: text/plain
    at com.ibm.rdm.fronting.server.services.publicapi.artifact.internal.ArtifactDataBuilder.adaptInputStreamUsingContentType(ArtifactDataBuilder.java:92)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...



Here is a flatten version section of the code I use:
                String structureURI = this.moduleURI + "/structure";
           
        HttpGet get = new HttpGet(structureURI);
        get.addHeader("DoorsRP-Request-Type", "public 2.0");
        get.addHeader("Accept", "application/json");
            
        HttpResponse responseGet = HttpUtils.sendGetForSecureDocument(clm_props.getServerURL(), get, clm_props.getCLMlogin(), clm_props.getCLMpassword(), httpclient, clm_props.getJazzURL());
      if (responseGet.getStatusLine().getStatusCode() != 200) {
           responseGet.getEntity().consumeContent();
           throw new HttpResponseException(responseGet.getStatusLine()
                 .getStatusCode(), responseGet.getStatusLine()
                   .getReasonPhrase());
        }
               
        Header etag = responseGet.getLastHeader("etag");
        this.moduleEtag = etag.getValue();
       if (clm_props.getDEBUG())
           System.out.println(">> moduleEtag \n" + this.moduleEtag);
     
        String moduleStr = EntityUtils.toString(responseGet.getEntity());
        responseGet.getEntity().consumeContent();

       HttpPut put = new HttpPut(structureURI);
        put.addHeader("DoorsRP-Request-Type", "public 2.0");
        put.addHeader("Accept", "application/json");
        if ( this.moduleEtag != null) {
         put.addHeader("If-Match", this.moduleEtag);
      }

       put.setEntity(new StringEntity(moduleStr, HTTP.UTF_8));
      HttpResponse responsePut = HttpUtils.sendPutForSecureDocument(clm_props.getServerURL(), put, clm_props.getCLMlogin(), clm_props.getCLMpassword(), httpclient, clm_props.getJazzURL());
      responsePut.getEntity().consumeContent();
       if (responsePut.getStatusLine().getStatusCode() != 202) {
           throw new HttpResponseException(responsePut.getStatusLine()
                 .getStatusCode(), responsePut.getStatusLine()
                   .getReasonPhrase());
        }
       
        /* ... */
                
Any idea what i am missing?
A header?