I cannot log into my Yahoo account, it says my password is incorrect

BusyCal or BusyContacts is unable to sync with your Yahoo account because it now requires that third-party apps use an app-specific password to sync with your Yahoo account. Your standard Yahoo password will not work.

Follow the instructions on this page to generate an App Password to use.

https://help.yahoo.com/kb/SLN15241.html

If you still want to login using your password without turning on Two-Factor authentication, you can visit this link:

https://login.yahoo.com/account/security#other-apps

and turn on ‘Allow apps that use less secure sign-in’. Yahoo does not recommended this as this may leave your account more vulnerable to compromise.

To learn more, please visit our help page:

https://help.yahoo.com/kb/SLN27791.html

Note: You might have to disconnect and reconnect BusyCal / BusyContacts to your Yahoo account after creating an App Password, as follows:

Select BusyCal / BusyContacts > Preferences > Accounts, click on the Yahoo account in the left sidebar, then click the minus (-) button to remove it.
Then re-add the account by clicking the plus (+) button, selecting Yahoo as your server type, and entering your Yahoo ID and the new app-specific password you created above.