Could not lock the device (Linux)

I have my Clip+ set in MTP mode.  Usually it connects to my Kubuntu laptop correctly and I can view the playlists.  But sometimes instead I get the messages “Could not read file” and “Could not lock the device” when I plug it in and try to view the Playlist folder.

I can clear the condition by logging out and logging back in again, but is there a less drastic way to clear the condition?  I imagine there’s a lock file somewhere – but where?