There are problems with calibre and having the library on networked/shared storage.
From the manual: "Do not put your calibre library on a networked drive."
You can avoid many problems by having your calibre library on a local hdd in a non-shared folder. And use the NAS for backups only. You can access the backups over the network.
There are many threads about this and causes and possible work-arounds. If you try using networked storage with calibre, make sure to keep your backups current and expect loss of data, possibly slow and undetected over long time so that errors/corruption may creep into backups.
https://manual.calibre-ebook.com/faq...rked-drive-nas