It maybe Onedrive. A recent edition of Windows 10 (possibly 1803 or 1809) defaults to having OneDrive synchronise the Document, Desktop, and Pictures folders to the cloud. Synchronising a calibre library to the cloud is a no-no, see ==>>
I am getting errors with my calibre library on a networked drive/NAS?
So you have two choices - keep the library in a location that Onedrive doesn't synch, or pause Onedrive when you're doing anything with calibre - including the command line tools, and the server etc.
As of version 3.30
Quote:
The default calibre library location is now not in My Documents but instead in the user home folder, to avoid issues with OneDrive auto-syncing calibre libraries.
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But that only applies to new installations of course.
BR