A little something to consider regarding @Arnie97's experiment with not clearing the DBs under /var/local. I went this route when I was downgrading my wife's PW2 (on v5.7.4) the other day. It was ok
initially. However, later when I tried to sideload a book it didn't show up on the home screen; it opened just fine in KOReader and on my PW2 so not a file issue. Figuring the only difference was that I didn't clear the DBs I thought I'd clear them now - did it and after a couple of reboots all was back to normal, sideloaded books would appear and all. So probably not a good idea to skip this step