As this was happening to me, my computer restarted and all notes that had had blank thumbnails (i.e. all those I created after upgrading to the latest evernote) were lost. Two days of writing work. Still hoping to resolve but imagine these notes are possibly lost permanently. If anyone has insight or possible fixes, desperate for them! The problem is ongoing with my Mac 2.4 Evernote.
Similarly, after upgrading to 10.2.4 on my Mac, Evernote was showing thumbnails of all my new notes but without detail. Upon restarting my computer two days later and rebooting Evernote, all of the notes I'd created since the update were gone. Still working to recover them - desperate for advice if anyone has insight here.