The API token's user is not connected to Salesforce (User must be connected to salesforce.), or the request used an admin-managed API key. A managed key is refused with This API does not support scoped credentials, or with This API does not support managed API credentials yet when it has the * scope. The Salesforce API requires a personal API token.

The write failed. When Salesforce rejected it, the body is the Salesforce result with success: false and the errors Salesforce returned. When Mixmax rejected it before calling Salesforce, for example an update without Id in the body, the body is an error with only a message.