https - webservice Consumer


I would like to configure a service in order to invoke an external WS through HTTPS. The endpoint WS is already defined through the HTTPS. I had exported the certificate from the web broswer and put in under mypackage/config in the system file.
I created an entry under the Web services in the web administration console:

Host Name or IP Address ourCustomerDNS
Port Number 80
User Name ******
Password ******
Private Key /opt/webm/webm71/IntegrationServer/packages/delme/config/exportedFormFirefox.der

From the Developer I created a service in order to load the file with the pub.file:getFile and read the certification as bytes. Then i connected the bytes directly to the auth/transport/serverCerts/privateKey.
At the WSD I set the handler in order to use the default WS Security Handler and the
Policy name:Consumer policy for username,Signature
Effective Policy name:Consumer policy for username,Signature.

After invoking the WS I’m getting an error:
Provided private key is invalid. ISC.0077.9016

It is obvious that something is not correct configured.
The WS can be accessed from SOAPUI and XMLSPY out of the box.