Hello Jürgen,
it seems to be related to the Homegear Binding. Took another Pi and used Mosquitto to transfer some (dummy) data via MQTT and for this source also rrd4j worked like it should.
This makes me quite wondering, as the data from the Homematic devices seem to be present in OH2. Even more: I could control the actors and also via REST API I see the state of the senors. Even with PaperUI I controlled and read out all HomeMatic devices.
However: as Homegear seems to be new setup in the unstable version, I’ll give OH2 in few months another chance. Up to this, old FHEM needs to work a bit longer.
Thanks for your nice and quick support.