Unable to connect Amazon RDS for PostgreSQL to Appflow

I get the following error when adding an Amazon RDS for PostgreSQL as a connection in Appflow, following these instructions: https://docs.aws.amazon.com/appflow/latest/userguide/connectors-amazon-rds-postgres-sql.html

"Error while communicating to the connector: Failed to validate Connection while attempting "ValidateCredentials with CustomConnector" with connector failure The request failed because the service Source Amazon RDS returned the following error: Details: Unable to connect to the database., ErrorCode: InvalidArgument. (Service: null; Status Code: 400; Error Code: Client; Request ID: null; Proxy: null)"

I can connect to the RDS instance pgAdmin, but for some reason I get this error when connecting on Appflow. The security group has All traffic set to anywhere for both inbound and outbound rules.

Are there additional requirements to connect to Appflow?