Since Java applets are no longer supported by the major web browsers for security reasons, I converted the most exciting applets I had created to Java Web Start applications. Such applications are supported by major web browsers such as Firefox, Chrome, or Edge, and while they don't integrate as easily and seamlessly as Java applets into web pages, they do have other advantages and - most importantly - can run safely inside a browser.
To setup and configure Java Web Start applications. All you need to do is to download and install openWebStart. Then restart your web browser and reload this page again. This time you should be able to start the Web App of your choice without problems.
If you still have problems, send me an email describing your problem.