“The requested resource could not be found.” error

Hi. We have a few forms on our site and recently, any time a submission is made, we first get the typical “Thank you for your message. It has been sent.” success message. But within a couple of seconds, this success message gets replaced with the error “The requested resource could not be found.” The submissions do get saved correctly, but this error message leads users to believe there was an issue with their submission. Do you know what might be causing this?
We previously had the plugins “Chimpmatic” and “Contact Form 7 Extension For Mailchimp” activated and had to deactivate them as they were causing issues. Could this be why we’re seeing this “Requested resource could not be found” error?

Thanks!!