No, it's normal for judges to teach courses on law. As someone studying law, I much prefer classes where a judge teaches or participates as a guest lecturer or panelist. Judges who are expert in a particular area of law often end up trying similar cases, and even if judges are randomly assigned (depending on the judicial system) a judge with established expertise will be noted mainly for his/her rulings in that area of expertise.
I have no idea about the merits of these particular cases or the quality of the judge's legal reasoning. It might be terrible. But Falkvinge isn't making legal arguments about why the judge is wrong, he's relying on people's ignorance of the judicial system to whip up opposition where none exists. Politicians always like to beat up on judges, and there is always a ready audience for populist bullshit.
In the original Dutch news report (linked to in the OP article), a third party lawyer observes that this was known about already but was not considered worthy of challenge. Recall again that the classes in question are aimed at lawyers; this is a very different situation than if they were teaching or consulting to copyright holders. Any legal class on copyright would also address matters like defenses to complaints of infringement and so forth.There's no reason to assume the class caters exclusively towards plaintiffs' attorneys.
All that said, I don't know anything about Dutch legal ethics in particular, and I might have misunderstood or mistranslated the information about the classes in question.
I have no idea about the merits of these particular cases or the quality of the judge's legal reasoning. It might be terrible. But Falkvinge isn't making legal arguments about why the judge is wrong, he's relying on people's ignorance of the judicial system to whip up opposition where none exists. Politicians always like to beat up on judges, and there is always a ready audience for populist bullshit.