在 Ionut Trestian 的帮助下，我找到了错误.我更改了配置以使用不同的签名服务，如下所示:

With help from Ionut Trestian I found the error. I changed the configuration to use a different signatureService, like the following:

{
    "dataSource": {
        "dataSourceArn": "arn:aws:appsync:eu-central-1:99999999999:apis/appsync-pi-id/datasources/Echo",
        "name": "Echo",
        "type": "HTTP",
        "serviceRoleArn": "arn:aws:iam::99999999999:role/roleName",
        "httpConfig": {
            "endpoint": "https://api-gateway-api-id.execute-api.eu-central-1.amazonaws.com",
            "authorizationConfig": {
                "authorizationType": "AWS_IAM",
                "awsIamConfig": {
                    "signingRegion": "eu-central-1",
                    "signingServiceName": "execute-api"
                }
            }
        }
    }
}

显然我没有正确理解配置值.在我的辩护中，我没有找到有关此选项的任何文档，只有一些散布在网络上的示例.:-)

Apparently I didn't understand correctly the configuration values. In my defense, I didn't found any documentation regarding this options, only a few examples scattered through the web. :-)