We had similar issues but not S3, it was with SFTP
I feel the S3 bucket that you configured does not have permissions or your credentials to connect to the S3 might be expired.
Try to connect to S3 bucket using curl command bypassing region id, secret key, bucket name, etc, and test the connection, if it fails through curl then there is a permission issue at AWS and not at the campaign side.
The 403 error talks about permissions, but I am wondering how the same s3 works for other workflows. is that other account also uses same S3 bucket?