"HTTP Error Bad Gateway", Sorry for asking again

I have seen this question asked multiple times, but without answers.
I installed the “All-In-One Monitoring Stack” for BigBlueButton according to:
all in one monitoring
It uses a docker for Prometheus and Grafana and the bbb-exporter.
When i try to add the datasource Prometheus:
http://localhost:9090
it show “HTTP Error Bad Gateway”
Where should i search for the mistakes i made?

Thanks a lot
Malte

Grafana 7.1.1

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