Please, could you provide the error message you get by calling the REST in the micrfoflow? You can check them in the consoloe of the modeler.
Are you having any log messages for us?
Could it be that you need to import the certificate first?
That is more a stack overflow question: https://stackoverflow.com/questions/18378869/pkix-path-building-failed-in-java-application
You probably did not import the certificate in the correct way.