Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
The error that you are receiving sounds like it is most likely related to "Constant Contact Forms Plugin for WordPress", which is not a 3rd party developed plugin, and is a WordPress plugin that is directly supported by Constant Contact's support team.
For direct assistance with the Constant Contact Forms Plugin for WordPress, you can reach out to Constant Contact's Support Team at 866-289-2101 and reference case #31227719 to be transferred to a specialist.
Please have a look and let us know if you have any other questions!