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Welcome to iPadForums, Pattysea!
Two more things to try:
If your devices are not up to date, update the software. The latest version is iOS 8.4. It's possible that an older version can produce this error.
Also make sure you've entered the right password. Sounds weird, but: enter the wrong password in Settings - Mail, Contacts, Calendar - your Yahoo account, then change back to the right password once more, let your iDevice verify the settings.
Did you set up the account manually, or did you use the Yahoo! option in Settings - Mail, Contacts, Calendars - Add Account to set it up?