Did anyone challenge the Nazi regime in court?

Question

In 1933, when the Nazi regime was not fully established, were there court cases where people tried to legally challenge illegal actions by the government (like imprisonment without verdict, removing non-Nazi and Jewish people from schools and universities, etc.)?

Answer 1

German Wikipedia says about resistance against "Aktion T4" (the killing of mentally disabled people, mostly children) in 1940/41

Als einziger deutscher Richter prangerte Lothar Kreyssig aus Brandenburg an der Havel die Euthanasiemorde an. Als Vormundschaftsrichter hatte er bemerkt, dass sich nach einer Verlegung Nachrichten über den Tod seiner behinderten Mündel häuften. Im Juli 1940 meldete er seinen Verdacht, dass die Kranken massenhaft ermordet würden, dem Reichsjustizminister Franz Gürtner. Nachdem ihm mitgeteilt worden war, dass die Mord-Aktion in Verantwortung der Kanzlei des Führers ausgeführt werde, erstattete Kreyssig gegen Reichsleiter Philipp Bouhler Anzeige wegen Mordes. Den Anstalten, in denen Mündel von ihm untergebracht waren, untersagte er strikt, diese ohne seine Zustimmung zu verlegen. Kreyssig, der damit gerechnet hatte, sofort festgenommen zu werden, wurde lediglich in den Ruhestand versetzt.

Schließlich sah sich der neu ins Amt berufene Reichsjustizminister Franz Schlegelberger genötigt, mehr als 90 höchstrangige Justizbeamte in die Vorgänge einzuweihen. In der Schlegelberger-Konferenz am 23./24. April 1941 gab er eine Stillhalte-Weisung aus, die widerspruchslos entgegengenommen wurde. Neue Ermittlungsverfahren wurden nicht mehr eingeleitet und ältere Verfahren niedergeschlagen. Damit waren die Juristen zu „Komplizen der rechtswidrigen Tötung“ geworden.

which Google translates to

Lothar Kreyssig from Brandenburg an der Havel was the only German judge to denounce the euthanasia murders. As a guardian judge, he noticed that after a transfer, news of the deaths of his disabled ward increased. In July 1940 he reported his suspicion that the sick were being murdered en masse to the Reich Minister of Justice Franz Gürtner. After he was informed that the murder operation was being carried out under the responsibility of the Führer's office, Kreyssig filed a murder complaint against Reichsleiter Philipp Bouhler. He strictly forbade the institutions in which his wards were housed from moving them without his consent. Kreyssig, who had expected to be arrested immediately, was merely forced into retirement.

Finally, the newly appointed Reich Minister of Justice, Franz Schlegelberger, felt compelled to inform more than 90 highest-ranking judicial officials of the events. In the Schlegelberger Conference on 23/24. In April 1941 he issued a standstill order, which was accepted without objection. New investigations were no longer initiated and older proceedings were quashed. The lawyers had thus become “accomplices in unlawful killing”

So we know one person by name (Kreyssig) who alarmed the police about ongoing murders, and that there where older proceedings to be squashed, which suggests that other people filed reports as well (and that the police did not think this was legal, even if starting with 1933 there were eugenics laws created that the killings could conceivably justified with. Had the police thought this was justified under the Enabling act or whatever, there would have been no need for clandestine orders to police officers).