Getting Error While Using P6 Web Services

we have an application which is using P6 Web Services to read and write the database , we have recently upgraded our landscape from R8.2.1 to 15.2.11 , in R8.2.1 application was working fine but with 15.2.11 it is throwing the error , although we have replaced all the lib with 15.2.11 lib.

Error received Error:com.sun.xml.ws.client.sei.SEIStub cannot be cast to rg.apache.cxf.frontend.ClientProxy

This part of the code is throwing the exception:

URL wsdlURL = new URL(getDbInstance().getUrl()+ getDbInstance().getObsService());

UserOBSService service = new UserOBSService(wsdlURL);

UserOBSPortType servicePort = service.getUserOBSPort();

//Error:com.sun.xml.ws.client.sei.SEIStub cannot be cast to rg.apache.cxf.frontend.ClientProxy