The format that is passed by the constructor is not being used here, thus causing errors when not passing the same format as the one that is automatically selected here. So, in contrary to the comment, I do believe that the format given here does matter. I have attached a patch with the proposed change.
I have noticed that in mason.sim.util.media.MovieEncoder.java:302-303: // i think my hacking begins here (dan)
// get formats with current framerate, which should not matter
Format format = (Format)(ImageToBuffer.createBuffer(i,frameRate).getFormat());
source = new MovieEncoderDataSource(format, frameRate);