Note for reading the paragraph below, Minna was Wagner's wife, though they were long seperated. Ludwig was King of Bavaria, Wagner's patron and a homosexual.
From Charles Osborne, The Complete Operas of Richard Wagner, (ISBN 0-306-80522-7) p. 139:
On the day in early spring 1865 that Tristan und Isolde was to have had its première in Munich by command of the King, Wagner received the news that Minna lay fatally ill. He also received a love-letter from Ludwig, who seemed to have cast himself as Isolde to Wagner's Tristan. In addition, Malvina Schnorr von Carolsfeld, who was to sing Isolde, developed a sudden attack of catarrh and lost her voice. To cap it all, at the instigation of Ludwig's ministers, who had begun to plot the composer's downfall, police officers entered Wagner's apartment with a warrant to attach his possessions for debt.