Hi gooseewo,
Hope this message finds you well. Reading through your situation here it seems like could be helpful.
Yahoo Help: Fix iCal errors, display issues, and buttons that don't work in Yahoo Calendar
There's a section in the article listed above that calls out Authentication errors such as yours. If you'd like you can follow the recommended steps below:
If you're unable to sync your Yahoo Calendar to a device or application, the server name or login info may be incorrect. To fix this issue, enter the correct server name in your calendar tools settings:
• Correct server name: https://caldav.calendar.yahoo.com
• Verify your login information works. Try to sign in to your Yahoo Mail account on your Mac
• If your password includes special characters(like '#' or '$'), change your password to exclude them, then update your password on your Mac, and attempt to reproduce the issue.
• If you're using an iOS device, try to sync your account with the Setup Wizard.
Give it a shot and let us know what you find.
Take it easy out there!