Similar issues happened a lot in recent several releases, at least to me, I posted replies in another thread
https://www.mobileread.com/forums/sh...0&postcount=11
The exact same issue of yours occurred to me too. I had to restore the backup of database file created by Calibre.
I haven't installed the latest firmwares(4.24.15672 or 4.24.15676) yet, don't know if there are more issues.