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Thanks both!
Funny that two different but similar species sat on the same plant (the only Candula in the garden).

Now, further questions arise:
I wouldn't be surprised if Matricaria would not be the only host plant genus of T. nigricauda, because M. chamomilla is almost absent and M. discoidea only sparsely distributed in the city.
That is of course under the assumption that it is in fact that species - because, Val, tell me if there is an invisible, but non-negligible 'cf' behind your ID, as I found this species-group mostly not to be IDed on species level, especially regarding my not-so-great photos.

Lastly, here are all the Tephritis species so far found in my tiny backyard:
- T. neesii (sitting on Leucanthemum)
- T. formosa
- T. praecox (see link above)
- one (yet) unidentified

An ID-request for a possible Dioxyna is still awaiting its confirmation