I had contacted the Webservices team at Constant Contact. Below is the reason why the application/plugin had stopped working.
Thank you very much for reaching out to us about this issue. I have been researching the cause and found that one of our representatives accidentally disabled the application while trying to go out of their way to provide a service to another customer who was having trouble.
While the intention was good, the result was definitely less than desirable and this is definitely an opportunity for training around applications and how to handle them properly.
At the time of sending this email, the application has been re-enabled and should be functioning normally again.
If you do see any behavior like this again, please feel free to reach out to us and we can check to verify if this is an issue on our end or not.