Which belief about the second law of thermodynamics did Max Planck change while developing quantum theory?

While developing quantum theory, Max Planck was forced to abandon his long-held belief that the second law of thermodynamics was an absolute law of nature.1

To successfully derive his radiation law, Planck had to relinquish this conviction and instead embrace Ludwig Boltzmann’s interpretation that the second law is a statistical law. This shift was a significant departure from his earlier work, where he had viewed the second law as an absolute principle alongside the first law of thermodynamics.1