I think I found the cause of the problem.
I suspect Lightroom Classic isn't able to move files that have certain extended attributes. When a file is residing in a local Dropbox-synched folder, the file is provided with a few extended attributes. Removing the extended attributes doesn't help because the Dropbox app apparently adds them again.
When a file is moved to a non-Dropbox-synched folder and the extended attributes are removed, Lightroom permits moving that file to the network drive (NAS). On the NAS the cloud synchronization (also linked to Dropbox) apparently doesn't add extended attributes, so moving them back and forth between the NAS and a local non-Dropbox-synched folder works normally.
Question: how do I proceed to maximize my chances that this bug will be fixed? Thanks.