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Thinking out of the box on this there is also a issue if there are any extra files in the captcha folder other than what was installed from the original zip. I had a similar problem two years ago. The ftp program I was using at the time wrote a log.txt file in the captcha folder and that broke it. Just deleting the extra files that don't belong fixed it.
Using IE, when I click on the X image, I see the reloading image arrow, but it continues to show the X image, however, I can't see the image or really do anything to reload it using Firefox.
I don't know if there is a setting in firefox to turn it on or off but it usually hides the old red x. The reloading thing happened to me also with the extra files issue.