Hi Incanus. On Likebook Mimas, Ares, Mars and Muses *with* firmware 1.x *any* recent version of KOReader should work. I would recommend stay with latest release version, which you can obtain from
https://github.com/koreader/koreader/releases
On recent FW like yours I don't know. Nothing changed in (our) platform code that could explain why is not working. Hence I would suggest to attach logs here and lets us know.
If you installed the version from Likebook store first and updated to a recent version *without* issues during installation that would mean that the old version wasn't modified and is signed with the same key. If that was your case I would suggest to uninstall the program, remove the folder /sdcard/koreader and install the latest version directly from github (or any version hosted by f-droid) and see if that works. If that's not your case then please logs
KOReader works from 4.0 onwards, but 4.2 is recommended because older android versions are a pain to maintain.