AWS API Gateway with custom authorizer returns Aut

Question

I have an AWS API Gateway that uses a custom authorizer, and if the request is authorized, it triggers another lambda function. Since yesterday, whenever I call the API, I get an error saying { "message": null } and a 500 Internal Server Error. In the response headers it says x-amzn-ErrorType →AuthorizerConfigurationException. I can see in the logs that the authorizer is called and returns a valid policy, and that the other lambda function is not triggered. I have not (knowingly) changed the authorizer. Can anyone give me a hint what might be wrong here? I have read this question but there the mistake was that the returned policy was wrongly formatted, while I didn't change my authorizer and it worked before.