Mqtt datasource: no data

i am nost able to get data from mqtt-datasource.

datasource itself is successfully connected. but when querying any topic i get absolutely no data.
mqtt datasource is a docker container, grafana is running in another docker container.
other datasource (influxdb in another docker container) is working as expected.

any ideas?

edit: unbelievable. fighting this issue for days i now found out, that this datasource is not wokring behind a reverse proxy (in my case nginx). any idea how to solve that? i expected that this connection is done by grafana-server and not the endpoint looking at grafana.