What it the fragrant, but moderately toxic compound in this plant?

The picture has been taken on 6th April 2019 at the edge of a woods on the Sissacherfluh (700 m) near Basel. Author and license: Wolfgang Schäfer, own photograph, public domain.

I will post the answer in one week.

Answer: The plant is Galium Odoratum, or Sweetscented Bedstraw, German name is „Waldmeister“. It contains the active compound Coumarin (German: Kumarin), which gives not only this plant but some cinnamon varieties and also hay its characteristic smell. The plant is used as ingredient of May Wine (German: Maibowle). It is advisable to limit the quantity and to use only plants before flourishing, otherwise the too high coumarin concentration can cause head ache.

Structural formula of coumarin = 1,2-benzopyrone = 2H-chromen-2-one.
Author and license of picture with formula: Emeldir – public domain