It is recommended to update to 3.6.4 as soon as possible, or delete example_form.ajax.php from your Securimage directory.
Additionally, version 3.6.3 adds support for the following:
- Add support for multibyte wordlist files
- Fix code generation issues with UTF-8 charsets
- Add parameter to getCaptchaHtml() to render components of captcha HTML individually for easier customization
- Fix database audio storage issue with multiple namespaces