Both manual and scheduled refresh of the Dataflow fails.
I'm able to edit the dataflow in Power Query, refresh and see the newest data.
The dataflow has been running for several months now and never had problems.
Somehow on Marts 29th it stopped working. I'm not sure if this is a coincidence, but this day we went from winter time to summer time i Central Europe.
I don't have any other dataflows, so I'm not able to tell if only this Dataflow is affected. Anyway because anything seems just fine in the online Power Query module I don't see any other options of debugging.