Fix authentication error in CalDAV or CardDAV
This error occurs either when your Yahoo login info isn't working or there's an issue with your device. Whenever you have an error we always recommend you to verify if your login info works. Sign in to your Yahoo account and if you're unable to sign in, learn how to fix sign in issues.
Use the iOS Setup Wizard
If you've added your account manually, instead try to use the iOS Setup Wizard.
Learn how to sync your Yahoo Calendar or Yahoo Contacts with the iOS Setup Wizard
Still getting the error?
If the issue persists and you're able to sign in to Yahoo Mail on a computer, then there's an issue with your device. Contact your mobile carrier for further help