[Adoby]

Remove calibre and reboot. Reinstall calibre again, the version you used before the upgrade. Configure the add folder, reboot and test again. Then upgrade and try the new version. Perhaps you have found a bug in calibre? But then I would expect many more users would react...

More likely that it is a problem with your PC.

What is the name and path of the folder? What are some of the names of the books?
If it is a long path, perhaps with accented chars, then that may cause problems.

Try the very simple and short folder path c:\calibre_add and the file test.epub

Make sure that the user that is running calibre has read/write access.

You can use the maintenance tools for you operating system to scan/check the harddrive/filesystem for errors.

After that all I can suggest is that you reformat the harddrive and reinstall windows from scratch. It seems that some part related to the filesystem of your windows install doesn't work correctly. Hard to figure out how to fix that. Easier to reinstall windows from scratch.