That does not seem to be a long term problem. Wind and solar can be down regulated with ease (and within fractions of a second), a negative prices only happen because producers got a flat-fee per kWh which is pretty much phase-out now. The problem is rather that Germany (plus Luxembourg) is still a single price zone, i.e when wind is blowing in Hamburg, the per-kWh price in Passau is also nill. While this is nice for Bavaria (the main culprit, as usual), there is an enormous cost for this in the form of re-dispatch fees as long as the grid is not strengthened a lot.