It partly depends how you read the accidentals at the beginning of each staff, and there are several manuscripts of this treaty and therefore of this Sobria. If one uses the PnD manuscript (from Paris National Library - Ex French Royal Library - Fonds Italien) as you do (I do not have access to something else anyway), and one makes the hypothesis that the two flats are one fifth apart, in the common order of B-E then the first three notes are D,D,D. It is consistent with the use of the C-clef in other parts of the manuscript, while strangely, no clef is written for this dance.
It is compatible with a Locrian mode on A :
A Bb C D Eb F G A, but somehow I doubt it. The fact that the clef was omitted on this particular air, leaves doubt on the copist's faithfullness or the convention used here.
It is also the only part of the manuscript I think where two flats are used (but look at the Proxenera which seems clearly in Bb).
I don't know of music in this mode for this period (around 1400-1450), especially italian authors, and Locrian mode is difficult to harmonize in several parts. I will ask around.