Behind the fall in nominal yields has also been a fall in real yields. This has been attributed to three main factors. The first is a secular slowdown in productivity growth across advanced economies, coupled with pessimistic expectations about growth potential in the years to come, which has reduced the expected rate of return on capital.
As far as I am aware, under German law there is no obligation for the federal government to recapitalise the Bundesbank, even when its losses are of such a magnitude that its equity base is threatened.