Without closer examination of the code I suspect that the return value from Maxima is misunderstood as an error message by the Maxima plot region. The image is actually produced but the error message is displayed instead. As the Draw2D() function doesn't have any special error handling, it is not affected by the problem. Should be not too hard to fix, yet, I have to upgrade my development environment, which I haven't touched for years. So for now it is safer to use the functions instead of the regions.
