The "403 - Forbidden" error in your HTTP connection within SSIS (SQL Server Integration Services) when downloading an Excel file from SharePoint is likely related to permission issues or security settings. Since the error message hints at problems with an "https" access, it's possible that the certificate being used is either expired, revoked, or not valid for the intended use.
Try to check the validity of the certificate being used in the connection. Ensure that it is not expired and is configured for the proper use case. If needed, renew or replace the certificate. If the connection is happening within an organization, coordinating with your IT department or SharePoint administrator might be necessary to understand if there are any specific security policies in place that might block the connection.
Additionally, verify that the credentials being used for the SharePoint connection have the necessary permissions to access and download the file. If all these aspects are in order, and the error still persists, you may need to explore more detailed logs or employ a network tracing tool to get more insights into the underlying cause of the issue, which might involve more complex network or server configurations.