Oh, yeah, +1, I also have a bazillion Noto variants on my OnePlus phone ;).

okey, the error happens if/when a system font was used for a document and now is disabled:

12-07 11:32:44.131 12016 12036 E KOReader: [NativeThread]: failed to run script: frontend/ui/widget/textwidget.lua:68: attempt to index field 'face' (a nil value)
12-07 11:32:44.131 12016 12036 E KOReader: [NativeThread]: Stopping due to previous errors

It should affect desktop linux too, but I didn't test.

a system font was used for a document
textwidget.lua:68: attempt to index field 'face' (a nil value)

That's not for a document, but for text in the UI elements - which should use only the NotoSans & Free* fonts we define in font.lua - which we still ship on Android.
So, the question is whether disabling system fonts make KOReader not use its own shipped fonts - or if, in spite of this disabling, it would still look for them, see them before the shipped ones, and ignore the shipped ones, and end up not using them because they are disabled (haven't look at the code, just some naive thoughts).

(Just checked how crengine behave when the main and the fallback fonts previously set disappear: it does not crash, and seem to use some Noto font.)

@poire-z: thanks for the info. I was stupid enough and didn't notice the error. This time with a debug build. As you can see the error is not related to noto, but with DroidSansFallback:

12-07 15:34:28.887 14143 14163 V KOReader:  Found font: NotoSans-Regular.ttf in ./fonts/noto/NotoSans-Regular.ttf
12-07 15:34:28.899 14143 14163 V KOReader:  Found font: NotoSans-Regular.ttf in ./fonts/noto/NotoSans-Regular.ttf
12-07 15:34:28.899 14143 14163 V KOReader:  Found font: DroidSansMono.ttf in ./fonts/droid/DroidSansMono.ttf
12-07 15:34:28.901 14143 14163 V KOReader:  Found font: NotoSansCJKsc-Regular.otf in ./fonts/noto/NotoSansCJKsc-Regular.otf
12-07 15:34:28.905 14143 14163 E KOReader:  #! Font  DroidSansFallback.ttf  ( DroidSansFallback.ttf ) not supported:  ffi/freetype.lua:142: Failed to load font './fonts/DroidSansFallback.ttf', freetype error code: 1

Reverting 503a57b fixes the error

The alternative to ressurecting all the fonts we got rid of :) would be to still pick "our no more shipped" fonts from system fonts, even when system fonts are disabled - which might be a bit tedious to code :|
But as we ressurected all of them except that DroidSans (did we?), I guess it's fine just getting it back and forget about that tricky font picking stuff.

Alright, let's just revert it. :-/ The Droid Sans one is pretty small anyway; it's big fat Noto I wanted to get rid of.