
Of course, I now have to install many more fonts in this way to test if the amount of fonts installed also plays a role. By right clicking I selected a font and selected "install for all users" and voilà, they were now also usable in MuseScore 3. This is probably where all user-specific fonts end up. It wasn't there! It was in "C:\Users\AppData\Local\Microsoft\Windows\Fonts". In the Control Panel, I selected one of these recalcitrant fonts and checked its properties to see if this font is actually stored under "C:\Windows\Fonts". After this action they were suddenly available again in Scribus, Inkscape etc., but still not in MuseScore 3. Strangely enough, there was no indication whether the fonts should really be overwritten, i.e. Then I copied these fonts with a file manager (Free Commander) directly into the Windows’ font directory (C:\Windows\Fonts).

Worse still, they weren't visible in Scribus 1.5.4, Inkscape 0.92.3 or Gimp 2.8.22, but in MuseScore 2 and LibreOffice 6 (of course also under Windows 10's Fonts dialogue). These fonts were again not visible in MuseScore 3. The fonts I installed were installed using the Fonts option in the Control Panel (drag and drop).

I have now researched again, using a new installation of Windows 10 (1809), on which only the system fonts, fonts of some programs and those fonts that I often use in MuseScore were installed.
