I am struggling to get cert-manager to issue certificates from let's encrypt which I can use with my ingress in Google Cloud Kubernetes.

I have a cluster running with a deployment, service and ingress in the default namespace. I also registered a domain at namecheap and added an A record to the IP address from my ingress. Now I can access the website with http and everything is fine. Now I want to move to https and things do not work.

I have installed cert-manager:

kubectl create namespace cert-manager
kubectl apply --validate=false -f https://github.com/jetstack/cert-manager/releases/download/v0.13.1/cert-manager.yaml

I verified that cert-manager is running:

kubectl get pods --namespace cert-manager
NAME                                       READY   STATUS    RESTARTS   AGE
cert-manager-5655447474-kw9k7              1/1     Running   0          99m
cert-manager-cainjector-59c9dfd4f7-fjzbf   1/1     Running   0          99m
cert-manager-webhook-865b8fb666-7kmx2      1/1     Running   0          99m

Now I created a ClusterIssuer kubectl apply -f letsencrypt-prod.yaml in the default namespace:

apiVersion: certmanager.k8s.io/v1alpha1
kind: ClusterIssuer
metadata:
  name: letsencrypt-prod
spec:
  acme:
    server: https://acme-v02.api.letsencrypt.org/directory
    email: 'my@email.com'
    privateKeySecretRef:
      name: letsencrypt-prod
    http01: {}

And added a certificate with kubectl apply -f certificate.yaml:

apiVersion: certmanager.k8s.io/v1alpha1
kind: Certificate
metadata:
  name: my-certs
spec:
  secretName: my-certs
  issuerRef:
    name: letsencrypt-prod
    kind: ClusterIssuer
  commonName: "www.mydomain.de"
  dnsNames:
  - "www.mydomain.de"
  acme:
    config:
    - dns01:
        provider: cloud-dns
      domains:
      - "www.mydomain.de"

This is my ingress:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: my-ingress
  annotations:
    kubernetes.io/ingress.global-static-ip-name: "my-ip"
spec:
  rules:
  - host: www.mydomain.de
    http:
      paths:
      - backend:
          serviceName: my-service
          servicePort: 80
  tls:
  - hosts:
    - www.mydomain.de
    secretName: my-certs

Now when I run kubectl describe certificate my-certs there are no events:

kubectl describe certificate my-certs
Name:         my-certs
Namespace:    default
Labels:       <none>
Annotations:  kubectl.kubernetes.io/last-applied-configuration:
                {"apiVersion":"certmanager.k8s.io/v1alpha1","kind":"Certificate","metadata":{"annotations":{},"name":"my-certs","namespace":"default"},"sp...
API Version:  certmanager.k8s.io/v1alpha1
Kind:         Certificate
Metadata:
  Creation Timestamp:  2020-02-23T13:30:15Z
  Generation:          1
  Resource Version:    787204
  Self Link:           /apis/certmanager.k8s.io/v1alpha1/namespaces/default/certificates/my-certs
  UID:                 a027a698-5640-11ea-bce8-42010a9c00dc
Spec:
  Acme:
    Config:
      Dns 01:
        Provider:  cloud-dns
      Domains:
        www.mydomain.de
  Common Name:  www.mydomain.de
  Dns Names:
    www.mydomain.de
  Issuer Ref:
    Kind:       ClusterIssuer
    Name:       letsencrypt-prod
  Secret Name:  my-certs
Events:         <none>

And in Google Cloud Console I see the message "Could not find TLS certificates. Continuing setup for the load balancer to serve HTTP" for the ingress.

What is wrong here or what am I missing?