I wonder if it could have something to do with the new dynamic permissions?
See
https://stackoverflow.com/a/37160573/2470572 and
https://github.com/koreader/android-...fest.xml#L7-L8
You should be able to test if that's the problem by going into Settings → Apps to give KOReader the relevant permission.
Other than that it'd be helpful if you could open an issue on
https://github.com/koreader/koreader/issues