Problem about soapRPC on HTTPS (IS 4.6)

I’m using WebMethods Integration Server version 4.6 SP3 on jdk 1.3.1.

I have created a Web Service Connector from the WSDL File in order to call an external partner webservice.

According with the content of the WSDL file, the Web Service Connector performs a soapRPC call on https.

Our partner gave us the private key and certificate to use for the https connection.
So, I have modified the Web Service Connector putting the call to the WebMethods service (in order to force the use of the our partner certificates) before the call
to the soapRPC service and putting the call to the WebMethods service (in order to revert back
to the default certificate) after the call to the soapRPCservice.

When I invoke the Web Service Connector, I get the error: Peer sent alert: Alert Fatal: handshake failure,
that it is raised by soapRPC internal service.

Any ideas or suggetions to better debug the SSL handshaking (e.g. to enable SSL handshake debugging in IS) ?