Just to clarify, are you trying to connect your Campaign Monitor Account under your Settings tab > Marketing > Email service provider > Campaign Monitor and is seeing the following error message:
There was a problem updating your email service provider. Please try again, or contact support if the problem persists.
If that's the case, could you try generating a new API key, as per Campaign Monitor's documentation and if you're still seeing the same error, could you capture a HAR file of the error and share it with me via direct message?