Problem

Turning on enforce_domain https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/#enforce_domain forces a redirect even of the localhost API calls done by the sidecars quay.io/kiwigrid/k8s-sidecar . It looks like when quay.io/kiwigrid/k8s-sidecar gets the redirect, it switches from POST to GET, at which point the request fails. Setting reloadURL manually is a workaround.

Suggestion

When setting enforce_domain, then default reloadURL to use root_url https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/#root_url instead of localhost. Which will involve some tweaking here:

Alternatives

I also filed grafana/grafana#93773 to try to make enforce_domain NOT redirect localhost (or have an additional option to do this) which would avoid the need to fix this.

Fixing quay.io/kiwigrid/k8s-sidecar to handle redirects is another option.