Thanks for your suggestion. I have Actual Window Manager (http://www.actualtools.com/?ref=aim66_prg&s=m) running, and that seems to cause the problem. Although I cannot yet explain exactly why it would happen, if I do not start Actual Window Manager the copy progress dialog has no problems at all. Luckily Actual Window Manager has a way to exclude certain programs, so I still can have the best of both worlds.
Thanks,
Frank