400 Bad request

Hello,

I use Mickael Cabanas "PrestaSoap" for PrestaShop and it works fine. Except the create function.
When trying to create a category via SoapUi, I get following error:

<SOAP-ENV:Body>
      <SOAP-ENV:Fault>
         <faultcode>400</faultcode>
         <faultstring>Other error : This call to PrestaShop Web Services failed and returned an HTTP status of 400. That means: Bad Request.</faultstring>
         <faultactor>PrestaShoapWSFault</faultactor>
      </SOAP-ENV:Fault>
   </SOAP-ENV:Body>

I dont know which forum can help me, I'm a little bit desperated.
Many thanks for your help!!!