When using the auto-responder modules in advanced mode, Joomla! cannot find the page to display in the iframe. This results in a 404 error or a "site within a site" presented in the module position where the form should go.
Solution: Upgrade to the latest Auto Responder Helper component.
See this blog post for details: Joomla Auto Responder Helper.
Also, you may try turning off the internal Joomla! SEF URL's in the configuration.