this error indicates a problem generating the captcha preview.
try deleting the Dreamweaver Cache file from the users configuration folder:
common_installation_issues.pdf
in the same configuration folder noted in the common installation issues PDF, find the shared/webassist/preview folder and delete that, it will be recreated.
if all else fails, you can try disabling the preview.
got to Edit -> WebAssist Preferences and check the option to disable the preview.