Filmed: 3-4 March 1975
Broadcast: Friday 4 April 1975, 17:40-18:20, ZDF, Germany
Shown in: Germany
Although there is a reference on IMDb, I suspect that this is not a unique ABBA TV appearance because it's not mentioned anywhere else and it is most unlikely they went to Germany just for one TV appearance.
Most likely it was just showing a PROMO clip, but as the broadcast date is 4 April, so NOT impossible when ABBA went to The Netherlands on 3-4 March they made a short trip to Germany too – but it need to confirm yet.
Cast (in credits order):
Thanks to Jozsef