I have a long-standing foxycart store that was linked with a Constant Contact list so people could opt in to the newsletter.
In investigating why it wasn't working anymore it appears that CC has changed their API so the old call doesn't work anymore.
The error in the FC admin reads "The requested api no longer exists."
The API being requested is http://api.constantcontact.com/0.1/API_AddSiteVisitor.jsp
I'm pretty sure I am using the most recent FC add-on files.
Here is the new CC API: https://developer.constantcontact.com/get-started.html
I know this was/is a third-party add-on. I'd love to hear any more info or suggestions on how this might be addressed.