We suddenly started to see 400 error when we try to update existing campaign using API call.
It had been working well for past years but it seems to have stopped working since a few days ago.
I would like to email our put request to your support team and would be great if you can contact me.
Thank you for reaching out to Constant Contact's API Support.
You can email us at webservices(at)constantcontact(dot)com. Please include the API endpoint you are calling, the payload, the API Key, and if possible the Constant Contact username in your email to us.