Issue
Attempting to connect using a valid Salesforce account and key results in the error:
Unable to connect to API. HTTP 400 Bad Request
Environment
Connector: Pardot
Answer
If you have multiple accounts in Salesforce linked to the same email address, you should instead enter your username in the Pardot account email field of the setup form.
Cause
When you use the email address, Salesforce will not be able to distinguish accounts and will return the error above. Entering your username allows Salesforce to identify the correct account when Fivetran queries the REST API.