Similar to problems garthbnyx encountered, I would like to provide more context and descriptions to the issue from where I encountered.
Recently, I was trying to re-organize my Evernote notebooks & notes in massive amount (I have 500+ notes categorized in tens of notebooks or stacks), something weird happens as shown in image below.
Two errors occurred:
Blank notebook
Duplicated notebooks
How they happened: These all happened in my `(C) Notes` stack. I was initially moving my notes between notebooks and it went fine. However, when I began moving Notebooks between Stacks, duplicated notebooks occurred (yellow & blue blocks). I thought deleting duplicated notebooks would work, so I emptied a couple of duplicated notebooks and deleted one duplicated notebook. To make things worse, the blank notebook occurred (orange block)(I guess it could be because I just deleted 1 duplicated notebook and left the other without a name, thus becoming a placeholder without an entity). Fearing more blank notebooks I would mess up, I just continued my re-organization and left more emerging duplicated notebooks as they were. Later when I checked my Evernote web client and Android version after sync, they both looked fine without errors.
What I tried: Thinking that my Evernote Mac version might be corrupted or messed up by my actions, I tried uninstall Evernote Mac along with all cached data & configurations removed using `AppCleaner`(if it really did).
Still, even after a reboot of my Mac and a reinstall, nothing has changed. Considering the web client worked fine. Failing to get things work normally on my Mac version, I switched my re-organization environment to web client. Until I saw garthbnyx's same issue mentioning Mac and iOS versions, I doubt if Apple's versions of Evernote are vulnerable to such errors.