Oficially PD4ML does not support HTTPS, however on practice it tries to load HTTPS resources with Java’s standard SSL classes.
WebSphere and Weblogic, for example, and may be some others implement their own SSL libraries, not derived from Java’s standard one. As a result it causes ClassCastException. As I see, it is not your case, but there may be other SSL protocol related issues, specific for your environment.
The best workaround for the HTTPS issue is not to use HTTPS for CSS or image references. It is expensive from the performance perspective and causes undesired network transactions. A better approach is to reference the resources with relative links.
If your source HTML documents are read-only and you are not allowed to change HTTPS URLs there, there is also a solution with custom resource loaders. You may implement your own one and to use any SSL library you like:
Also http://pd4ml.com/examples.zip include source codes of SSL servlet, where you could borrow some ideas from (for example how to suppress a certificate check).