I installed KOReaer and Plato following the instructions of this thread, using Linux Kubuntu 20.04: In my Kobo Aura One, last firmware, when I use Plato and I return on Nickel it is impossible reading any epub untill a fire off My Kobo and light on again. It claims that the book is DRMed and I don't have the right to read it. I have no DRMed books now in my Kobo.
It only happens with Plato, with KORead it is all OK, and it happens only with Epub, Kepub are regularly opened.
Well, I am not a great user of Plato, it is only for curiosity that I installed it and I can uninstall it and repeat the installation only with KOReader (oh thank You for having it in Kobo Menu without the explicit installation of NickelMenu, it is a fantastic idea)
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Anyway since it is certainly not the expected behavior I think it is better to let you know. I can't know if it is Plato itself that causes the issue or it is KFMON after using Plato. May be also NM after Plato? Can You check it to see if it is only my Kobo that experiments the issue? May be some patch conflicts with it? (but I remember that a few months ago You said that this installation is not influenced by patches.)