Why is suddenly this java security warning shown on our applet

A customer got this Java security warning on one of our ancient java components on a web site we develop:

The java applet is a tiny applet that does very little (it is simply a dropdown with the ability to write freetext). Is it a simple way to avoid this security warning?

The applet is something we have developed a long time ago, which has been running in our web solutions for ye开发者_如何学运维ars.

---

Sun/Oracle changed something in Java SE 6 Update 19 and up in relation to mixing signed and unsigned code. The linked article has tips on solving this.