I kept getting this error in a class which allows videos to be played. Eventually, I managed to track the error down to the following line:
VideoControl videoControl = (VideoControl)(player.getControl("VideoControl"));
I have the exact same line elsewhere in the same MIDlet and it works fine, so I don't understand what the problem is, and why, of all things, it would result in a certificate error. I have a Verisign code signing certificate which works fine in the absence of this line. Any ideas?