I just came across this problem. I figured out that at some point I uploaded files to my Windows web server from my Mac. When I did this, the Mac created a set of hidden files in the includes/captcha/plugins folder:
These files should all be deleted. Each of these has a corresponding file which should remain, but all the files starting with ._ need to be deleted. If your server is Windows, you'll need to enable hidden files in order to see them. If your server is Linux, use ls -a to see these files.
Probably too late to solve the original poster's problem, but maybe it will help some googlers out there.