The Mac version of Evernote kept telling me, "Your password seems to have changed. Please log in again." - even though I didn't do anything. So I went to the web site and logged in with my old password, and it told me to change my password. Once I did that and updated all my clients, everything started working again.
I didn't know what happened until I saw this, but I figured it was something it was something like that.
Edit: Evernote has released minor updates to many of its apps. They probably provide clearer explanations now.