Sorry I don't speak German, I'll have to write in English.
This error (empty remote calendar), usually occurs when you select a remote calendar in Thunderbird, and that BirdieSync finds no event in it. At the moment, BirdieSync cannot make the difference between a problem in the loading of the remote calendar, or if the calendar is simply empty (no event has been created yet in the calendar).
So create an event in this calendar, if you still get the same error while synchronizing, it means that Lightning has a problem accessing to this remote calendar and doesn't succeed in loading the calendar.