you do not have permission to read the file

I use Calibre (v. 8.16.2) to manage my ebooks on my Kindle Paperwhite. Recently, I uploaded a bunch of epubs and most were accepted, no problem. But about a third of them gave me the error:
Could not add *location*\name of book.epub as you do not have permission to read the file
I can open and read the book in Windows file explorer, so I know it's not a drm issue. I could understand it being a kindle issue if all of my epubs suddenly couldn't be loaded. But I just don't know how to solve this issue.