Austropa Interconvention on "[Plugin: Email Before Download] Error [contact-form 404 "Not Found"]"

Any time we try to activate "Email Before Download" we keep on receiving following error: [contact-form 404 "Not Found"]

Or "Contact Form 7" Version is 2.4.2
The Download Montor is also installed.

Do you have any idea, what this error tells us?
We would really like to use this great plugin, as
it exactly does, what we need.

Thanks for your help!
Alfred

http://wordpress.org/plugins/email-before-download/