Typically when a judge makes an error of law, it's appealable. Such an error, even a very bad one, does not usually give grounds for removal for bias.
In this case, the Judges role is to hear the case and form views on each sides presentation, and the evidence - 100% of which the Judge already has on paper - and all the critical fact witnesses heard. To my mind, the fact that the Judge, at this point, has reached the view the case does not support the warrant anymore, is a finding of fact & law that the Judge is entitled to reach.
However if that decision is legally wrong, the prosecution will no doubt successfully appeal it.
Personally I will be highly surprised if the Judges are removed in the middle of the trial.
MOO - not a german lawyer.