Who has stopped incorrectly?

You were right: only the red car has stopped incorrectly.

In Germany you must not stop or park on a pedestrian crossing (zebra crossing) or in a way that blocks it. The reason is visibility and safety: pedestrians must be able to step onto the crossing, and approaching drivers must be able to see them clearly. The red car is positioned right at/over the crossing area, so it obstructs pedestrian access and sight lines, which makes the stop illegal.

The yellow car is not stopping on the crossing and does not block the crossing area, so its stop is allowed.

Rule of thumb for similar questions: if a vehicle is on the zebra crossing or so close that pedestrians can’t use it freely or can’t be seen in time, it’s considered an incorrect stop.