Nice to meet you, this is Juan Rosales from ContactUs.com
Thank you for downloading our plugin. In order for me to help you the best way, I wanted to ask you to give me more details about the error you get. Is it:
a. When you finish configuring your form and try to save it you get this error?
b. or you have already implemented the form on your site and whenever you try to submit a lead you get this error?
Either way you might want to check your form settings one more time, save and publish them. In our Youtube Channel http://www.youtube.com/contactus.com you can find many tutorials on how to configure and implement the form on your site.
Let me know if that works and I will wait for your comments.
JJ- ContactUs.com Support