I just looked up and learnt there’s so many different versions of Kobo! Am I understanding this right?
Kobo Clara 2E + 2.5.ttf 
Kobo Clara 2E + 2.6.ttf 
crashes
Kobo Clara 2E + 2.6.otf 
does not crash but falls through to some other fonts
Kobo Clara 2E + 2.7.ttf crashes
Kobo Clara 2E + 2.7.otf crashes
If yes, then looking back at the changelog, it is probably one of the following being the culprits:
- Cleaning up zero-width non-joiner (removing parts for this one glyph)
- Bodies for phantom glyphs (adding paths for all the phantom glyphs)
The strange thing is that these two are essentially opposite operations (albeit on different set of glyphs) 
. When poohsticks can talk about his Kobo version then we may be able to pin down a little more.