Why does Jam API for creating tasks doesn't work?

In S&OP we're using the Jam REST API for creating tasks. It returns HTTP code 201, but the response body is empty and when we open the JAM UI we don't see the new tasks. Our code is working well with another Jam instances. Here's an example (with an actual data):

REST POST

URL: https://jam12.sapjam.com/v1/activities/10725/action_items

Headers:

Content-Type   application/xml

Authorization    OAuth <token>

Body:

<?xml version="1.0" encoding="UTF-8"?>

<action_item name="1-st step STEF">

  <title>1-st step STEF</title>

  <description>Please complete the task for the process step "1-st step STEF". This task is part of the process "STEF 11/07/2014" and is automatically generated as part of the SAP Sales and Operations Planning process model.</description>

  <due_date>2014-11-09</due_date>

  <assignments>

  <assignment>

  <participant email="sfAdmin@successfactors.com" />

  <status value="none" />

  <note />

  </assignment>

  </assignments>

</action_item>

Seems like a bug, reproducible with certain Jam instances. Can you help me with this?