GrafanaMimir Alertmanager Datasource not working

[MarcelGlaub]

Hello everyone, I have problems with creating Mimir alerts.
My setup:
MinIO VM as S3 storage backend
One Kubernetes Cluster with Prometheus remote writing metrics to Mimir
A second Kubernetes Cluster running GrafanaMimir and Consul

Grafana v10.0.3
Mimir 5.0.0

First of all, I have my normal datasource via which I receive the metrics from Prometheus. The datasource works for querys and visualizing metrics in grafana. As soon as I go to AlertRules i get an error saying Failed to load the Datasource configuration for Mimir: Failed to fetch but the Datasource is still there for selection:

Then I actually have an Alertmanager Datasource, configured like this:

    - name: MimirAlertmanager
      type: alertmanager
      url: https://mimir.k8s-internal.datacenter.de/
      access: server
      jsonData:
        implementation: mimir
        handleGrafanaManagedAlerts: false
        tlsSkipVerify: true
      basicAuth: true
      basicAuthUser: <user>
      secureJsonData:
        basicAuthPassword: <pw>

Health check passes but the datasource is not displayed in AlertRules. If I put /alertmanager after the url I get the following error during the health check: It looks like you have chosen a Mimir or Cortex implementation, but detected a Prometheus endpoint. Please update implementation selection and try again.
But then when i put /alertmanager after the url and change the Implementation to Prometheus the HealthCheck Passes again.

So to summarize: I don't understand how the config should look so that a datasource is displayed here in grafana as a Mimir datasource

I have tried what feels like all the possibilities. I also find the documentation quite confusing. for example: The documentation says to use the /prometheus prefix:
https://grafana.com/docs/grafana/latest/alerting/alerting-rules/create-mimir-loki-managed-rule/
and in turn it says that you should use /alertmanager:
https://grafana.com/docs/mimir/latest/references/architecture/components/alertmanager/

I can't see any errors in the grafana/ruler/alertmanager pod logs. I have configured ruler api, and external urls as described in the docs.
My value files:
mimir_values.yaml.txt
grafana_values.yaml.txt

I would be very grateful for suggestions 😃

Cheers
Marcel