We didn't know that. Furtwängler conducted Tristan und Isolde at the Frankfurt Opera, as guest conductor. This was in June 1921 — when exactly? — before the 28th. The only evidence of this, at least for the moment, is an insert in the evening edition of the Hamburger Fremdenblatt, dated Tuesday, June 28. Recall that Furtwängler had been the leader of Frankfurt's Museumkonzerte since 1920.
Apparently it was a success, also because of the presence, in the role of Isolde, of Béatrice Lauer-Kottlar (1883-1935), who later became better known as Sutter-Kottlar after her marriage to Otto Sutter, a pressman. After Strasbourg ( did she meet the young Furtwängler there?) and Karlsruhe, she had been based at the Frankfurt Opera since 1917, where she performed major dramatic roles: Donna Anna, Leonore, Brünnhilde, The Marschallin...
Does anyone know more about it?
