After the death of Ulrich III Sonnenberger, the Archduchy of Austria and the Archdiocese of Salzburg again had an argument over who was entitled to appoint a new Bishop of Gurk.

Emperor Frederick III would have preferred the appointment of Lawrence of Freiberg, who was Provost of Gurk at the time.

Both candidates were summoned, under threat of excommunication, to come to Augsburg, where the dispute was settled by Cardinal Mark, who was Patriarch of Aquileia and Papal Nuncio to Germany.

Sixtus was chancellor of the Prince-Bishop John IV Tulbeck of Freising, who resigned in 1473 in favor of Tannberg.

Tannberg was very pious and led to several diocesan synods and introduced profound reforms of the clergy in his diocese.