PalmenGarten Blüte

It looks very much like Winter Aconite. They've just started blooming here in Chicago (buds are ready to open, but not yet open). How's the weather in Frankfurt am Main versus Chicago?

Yes. It is Winter Aconite (Eranthis). The usual one in cultivation is Eranthis hyemalis.

Yes, that's a winter aconite (Eranthis, I'm not sure what species). It's a close relative of buttercups (Ranunculus). One way that the relationship's obvious is the distinctive glossy gleam on the yellow petals. At least around here, that makes buttercup-kin unmistakable, although I suppose that somewhere in the world there may be other plants that have independently come up with the same shiny petals.

Well you won't go wrong with "Winter Aconite", but hyemalis is not the only possibility- although it is the only one of the eight or so species of Eranthis native to Europe, there are many in cultivation

Eranthis albiflora is, as the specific epithet suggests, a white-flowered species, as are E. pinnatifida, E. sibirica, and E. stellata...

although yellow-blooming, E. longistipitata has needle-like leaves as so would be out of contention here...

however E. cilicica is very similar to hyemalis and although native to southwestern Asia, is quite popular in European bulb gardens... hybrids between hyemalis and cilicica (I think labelled as E. x tubergenii) are also widely available to alpine gardeners and are yellow too...

Eranthis albiflora

Eranthis cilicica

Eranthis stellata

Eranthis x tubergenii

can't think of any more but as hyemalis and cilicica are often sold as the other, and as Frankfurt is really considered northern Europe and therefore out of the native range of hyemalis, I suppose it could be either or even the hybrid...

(the eighth species is lobulata from western China, but I have never seen it)

In German: Winterling.