This … was … a … pain. Sure, most forum commenters said that to make this work, you will have to have the attribute Trusted-Library: true in the manifest of your jars and third party jars that your applet uses.
However, that is only half of what needs to be done. Even though all my third party libraries had that attribute in the manifest and signed them I never figured it out until so many days later.
So what is the solution?
Make sure all your jars have Trusted-Library: true in the manifest.
Inside your main jar, add another attribute Codebase: *