Welcome to the club! You're not the first one who tries to upgrade the 360+ firmware to the 360 reader. It doesn't work. Look here for instance:
https://www.mobileread.com/forums/sho...d.php?t=147627
I don't know if there is already a solution. Maybe you can send a PM here to Forkosigan (
https://www.mobileread.com/forums/member.php?u=37926) from Pocketbook. If anyone can help you, then he.