Perhaps you should search on this forum for messages about not using calibre on a network share? Or read the calibre FAQ titled:
I am getting errors with my calibre library on a networked drive/NAS?.
You may also want to take a look at
SQLite Over a Network, Caveats and Considerations for SQLite's take on doing so
Not to mention that calibre 6.4.0 is an old version so at least update to 6.29.0?