I believe this problem may have something to do with a hardware/device check on the device you are using -- in this case, a Mac with a device UUID. I was getting this exact error. Since I was using two factor auth on Mac OS X 10.9 (on a Retina 15) I thought I would turn off two factor auth. No go. Since I could log in with other computers running 10.8.x and devices, I figured it was device specific. I deleted the app and reinstalled. I killed the data in ~/Library/Application Support/Evernote/*. I used AppCleaner to make sure all was gone. No solution. So I turned on the log and watched as I attempted to log in with my known good email and password. Here is what turned up: 2013-11-04 20:14:44 main [ENVideoLoginWindowController] INFO: Making login panel key and front.2013-11-04 20:14:46 [ENAuthenticator] INFO: Authenticating email@example.com/Evernote to www.evernote.com2013-11-04 20:14:46 [ENAuthenticator] DEBUG: -[ENAuthenticator doAuthentication] authentication error. hardwareUUID:3********************************************3computerName: 2013-11-04 20:27:02 [ENAuthenticator] INFO: EDAMUserException(errorCode:2,parameter:"deviceDescription")2013-11-04 20:27:02 [ENAuthenticator] INFO: Unknown EDAMUserException. [Timestamps are off because I copied a previous failure... ugh] And then it gave the dreaded error noted in the title. So I went into System Preferences and renamed my laptop (because my device -- in this case a MBPR15) had a null value (a space) for Machine name (this was on purpose). So I gave it a new non null name, and the app immediately logged me in and requested a sync of my notes, which finished properly. There is a device auth going on and it definitely doesn't like a no name value.I'll turn two factor auth and report back if I encounter this again or other.Hope this helps people out.