add task to a project using PSI manually

Hi

I try to add a task to a project via Nintex Workflow by calling the PSI.

Therefore I cannot use the methods described in msdn and have to build the XML structure manually.

I try to use the QueueAddToProject method with this XML.

<diffgr:diffgram xmlns:diffgr="urn:schemas-microsoft-com:xml-diffgram-v1" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
<ProjectDataSet>
<Task>
              <PROJ_UID>678030fd-ca99-4664-9ebd-6bc7b6a93e39</PROJ_UID>
              <TASK_UID>2aa3268b-4f57-46e0-b281-c88c682eb001</TASK_UID>
              <TASK_PARENT_UID>8e50f4c3-baca-41d5-a777-f790e30ec713</TASK_PARENT_UID>
              <TASK_NAME>new task</TASK_NAME>
     <TASK_DUR_IS_EST>true</TASK_DUR_IS_EST>
              <TASK_DUR>4800</TASK_DUR>
              <TASK_DUR_FMT>39</TASK_DUR_FMT>
            </Task>
</ProjectDataSet>
</diffgr:diffgram>

When I send it, I get a GeneralSecurityAccessDenied regarding categorypermissions.

I have ALL permissions (yes, I am sure.. :-)

So I fear it is perhaps something regarding the XML I send?

Thanks for help

Christoph