Tutorial verbatim fails with Missing Authentication Token

I just ran through the tutorial at https://docs.aws.amazon.com/lambda/latest/dg/services-apigateway-tutorial.html precisely as instructed. At the first test hitting the public URL it fails thus:

$ curl https://9999999999.execute-api.eu-west-1.amazonaws.com/test/DynamoDBManager \
-d '{"operation": "create", "payload": {"Item": {"id": "5678EFGH", "number": 15}}}'
{"message":"Missing Authentication Token"}

What I checked:

• The DB is in eu-west-1
• At the gateway, POST has Authorization=None and API Key=Not required
• All the preceding tests worked
• Under API Gateway -> Stages -> test -> Deployment history the deployment has Status=Active
• The policy is as instructed and referenced by the role which is referenced by the lambda function under Configuration->Permissions