I could reproduce this by using today's latest-qe images from quay.io.

When accessing Grafana, everything seems to work until you open a dashboard:

Templating init failed
Bad Gateway

Grafana tries to talk to http://prometheus:9090 which is an HTTPS server, so it's confused:

2019/08/09 16:40:07 http: proxy error: net/http: HTTP/1.x transport connection broken: malformed HTTP response "\x15\x03\x01\x00\x02\x02"

The configmap looks like this - it seems to be missing the basicAuthPassword apart from the wrong scheme

datasources.yaml: |
apiVersion: 1
datasources:

• access: proxy
  editable: true
  isDefault: true
  jsonData:
  timeInterval: 5s
  name: Prometheus
  orgId: 1
  type: prometheus
  url: http://prometheus:9090

Something seems to be wrong with the patchGrafanaConfig in the operator