I am also seeing this error. There is limited info when doing a web search.
Running on FC6 with the latest and greatest of everything (JBoss, Java, etc.)
I would guess that you have deployed an application or service that needs this class. The class is found only in server/xxx/deploy/management/console-mgr.sar/web-console.war/applet.jar. Thus the class is available only to the web console. If you need it for your application, try adding that jar to your war, ear or sar file.
This was a pure install on Fedora Core 6 with the latest JBoss. Once I reinstalled the JavaSE JDK everything seemed to work.
It must have been some sort of class path configuration error on my FC6 setup.
Also, the information at the link below was crucial in my reconfiguration:
The information at this link should be part of the J2SE install instructions for Linux.