Thanks for your help, Austin. Having the exact same problem.
1) I am able to log in successfully, but haven't been able to sync due to user name/password error yesterday or today.
2) I can log in and sync properly using the same user name and password on the web portal, an IOS device and also on a different Windows computer.
3) Signing out and in does not help.
4) NOTE - the problem is with is an Evernote for Business account. I am able to access and sync my personal Evernote without a problem using the same computer that can't sync my business account .