Problems with indexing have been encountered/discussed a couple of times
https://www.mobileread.com/forums/sho...d.php?t=135099
https://www.mobileread.com/forums/sho...d.php?t=127047 from message #14 onwards
https://www.mobileread.com/forums/sho...d.php?t=139330
https://www.mobileread.com/forums/sho...d.php?t=141254
but this is very technical stuff
If I remember correctly (maybe wait for one of the experts to confirm) problems could come from a corrupt global.db file.
What I would do: re-install latest firmware and latest DR800+ (v4), disable "automatic thumbnail generation" in settings, reload all your books
But again, maybe wait for one of the experts for a less radical solution ...