If you are running calibre on a library, there is a very good chance that it will be modifying the database.
As for asking on the site about running calibre with a library stored on DropBox? I'm sorry but my experience with the posts I read since I started on MobileRead is that using any type of network attached storage for a calibre library makes it a matter of
when not
if your library will be corrupted which makes hard to see how anyone in the possession of a moiety of their mind would have told you that doing so is okay.
Run calibre with your library on local storage. Use a one-way sync to copy your library to a cloud storage. That I've seen recommended. Your suggestion about running calibre with the library stored in the cloud? That I have never seen recommended.
Did you even read the information at the two links I posted?