To Burckhardt’s and Ritschl’s consternation, Nietzsche tried to co-opt the Schopenhauerian aesthetic-metaphysical mysticism in his first ‘historical’ work, The Birth of Tragedy (1872). “But our Nietzsche!” Ritschl would write to Wilhelm Vischer, the man who a few years before hired Nietzsche at Basel, “It’s remarkable how in one person two souls live next to each other. On the one side, the strictest method of academic scientific research…on the other this fantastically-overreaching, over enthusiastic, beat-you-senseless, Wagnerian-Schopenhauerian art-mystery-religion-crap [Kunstmysterienreligionsschwärmerei]! […] What really makes me mad is his impiety against his true mother, who had suckled him at her breast: philology” (KSA 15, 46f).
Schwarmerei, I know, is the German counterpart for the old early modern derogatory sense of 'enthusiasm': it suggests an overheated imagination. So Kunstmysterienreligionsschwärmerei would indicate the madness of one who treats art as a mystery religion.