NoCredentialsException while calling web service method

I'm trying to fetch data from Integrity via web services. To get a better picture of the data (), I use SoapUI to call the web services.

However, every method I call fails with NoCredentialsException as answer. I have the needed rights on the project, my request envelope for a call of "getSubprojectList" looks like this one:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:sour="http://webservice.mks.com/2009/7/Source" xmlns:sch="http://webservice.mks.com/2009/7/Source/schema">

   <soapenv:Header/>

   <soapenv:Body>

      <sour:getSubprojectList>

         <!--Optional:-->

         <arg0 transactionId="">

            <sch:Username>user1</sch:Username>

            <sch:Password>xxx</sch:Password>

            <!--Optional:-->

            <sch:ImpersonatedUser>user1</sch:ImpersonatedUser>

            <sch:Project>/repository/test.pj</sch:Project>

         </arg0>

      </sour:getSubprojectList>

   </soapenv:Body>

</soapenv:Envelope>

Here is the complete response soap envelope:

<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">

   <env:Header/>

   <env:Body>

      <env:Fault>

         <faultcode>env:Server</faultcode>

         <faultstring>Unable to get subprojects of "/repository/test.pj".</faultstring>

         <detail>

            <ns1:MKSException class="com.mks.api.response.NoCredentialsException" implication="Unable to get subprojects of &quot;/repository/test.pj&quot;." xmlns:ns1="http://webservice.mks.com/2009/7/Source/fault">Authentication failed for user1 on mks1:9001</ns1:MKSException>

         </detail>

      </env:Fault>

   </env:Body>

</env:Envelope>

Anyone has a hint, what am I missing ?

Thanks in advance, Hendrik