Recently I came across a problem to use the function "open book folder" which did not work to open my Dolphin file manager for any selected book.
First I thought it was caused by an access problem due to flatpak, snap install. But finally I realized it was based on an insuficcient association of the file type "inode" in the KDE Plasma settings.
Solution: change file type association settings to another application > apply > change back to Dolphin > Done, you should be fine now!
detailsand help can be found under:
https://userbase.kde.org/System_Sett...ssociations/en
Have fun using calibre!