I am getting this error too. My applet uses arduino, serial, and tweet_stream libraries. Has anyone found the solution for this? I am not sure if I can control Arduino from a web embedded applet anyway...

Issues with applets exported with OpenGL are very frequent, search the forum for that.

And indeed, stuff involving hardware and native libraries (including OpenGL...) is hard to put on the Web, because of the low-level system access they need, which is prohibited, by default, by Java & the browser.
This can be overridden by signing the applet.

But honestly, what is the point to export an applet using the serial library? The chances that the person launching the applet has the appropriate hardware is near zero, anyway... If they have, they probably have Processing, so publishing the sketch in source form is probably enough.