Maybe someone can give me a hint. Trying to export all layouts in compressed MusicXML-format leaves out some layouts (see attached screenshot). What could be the reason for that?
I suspect the issue is that you have some characters in the layout names that the component that writes the zip file is choking on, e.g. the ampersand in your PRIMO & SECONDO layout, and the vowel with an umlaut in your full score’s name. This is an area we need to fix up. Given that the MusicXML file is far from exhaustive anyway you are probably no worse off using the uncompressed .xml format anyway.
Thank you Daniel for the quick reply! My usually do my exports in uncompressed format, just wanted to test. So I can live with that.
But: where is the vowel with umlaut? Maybe I am a little short-sighted this morning 
I think I need to clean my glasses: I thought it said “Gesämtpartitur” (which no doubt exposes my ignorance of German!).
LOL