Clara HD Utlimate manga reader or else ?

Hello everyone,

I've got an old Kobo Clara HD but it still works great.
Launcher used : NickelMenu
Manga app installed : Ultimate manga reader

Problem : once closed, my Kobo reboots.

Btw, is there a way to add sources for mangas (French or other languages ?)
Is NickelMenu still a good launcher, are there better ones to try ?

thanks :)