How do I fix a Gravity Forms 'Bad Request' subscriber error?

Hello, we use Gravity Forms with the Constant Contact integration. Everything has been setup correctly and submissions often work - however recently we have been seeing a 'Bad Request' error with the Constant Contact portion of the submission and the user isn't being added to our List:

I can't find any documentation on this - or why it may happen. I am not sure if it is an anti-spam mechanism that is flagging honest submissions or what. Any thoughts? I have also reached out to the Gravity Forms support, though it feels like a refusal from the CC side of things.

Thanks!