Seemingly also the Wikipedia article (https://en.wikipedia.org/wiki/Ultraviolet_catastrophe) claims the pop-sci ordering: “As the theory diverged from empirical observations when these frequencies reached the ultraviolet region of the electromagnetic spectrum, there was a problem.[3] This problem was later found to be due to a property of quanta as proposed by Max Planck: There could be no fraction of a discrete energy package already carrying minimal energy.”