"Herr Dandolo", comic opera after an Italian play by Rudolf Siegel, was first performed in 1914 and appears to have been quite popular for a short time. It got generally favourable reviews. The vocal score with piano is available in the Internet Archive (https://archive.org/details/herrdandolokomis00sieg/mode/2up). Siegel was a pupil of Engelbert Humperdinck and Ludwig Thuille.
Thank you for the link to Herr Dandolo! Here is another piece by Rudolf Siegel in handwritten full score, the Minna von Barnhelm Suite after the comedy of Gotthold Ephraim Lessing:
https://daten.digitale-sammlungen.de/~db/0013/bsb00139224/images/index.html
It is part of a collection of Siegel's manuscripts in the Bavarian State Library in Munich.