I would suggest checking the settings of your antivirus software to see if it is scanning or blocking access to your book files. Also, if you have any file synchronization services (like Dropbox, Google Drive, or OneDrive), make sure they are not actively syncing the folder where your books are stored, as this can sometimes cause access issues.
Another solution you might want to consider is using a dedicated eBook management tool. For Windows 10, a user-friendly option is
https://bibliovore.en.softonic.com/. This software is designed for managing and reading eBooks on Windows devices, and it can offer a smoother experience with your Kindle Fire HDX books.