SOLUTION: It appears that some services require the use of an “app password” to login. You create the app password using a web session with your service provider. It will require that you login and then choose from a list of apps (usually eMClient is not there, but you can add it) and the 16 lower-case character password will be generated. It may be shown with spaces for readability. Copy or write down this password.
In eM Client, choose Menu>Accounts, pick the account that matches your service provider, then select the CalDAV tab. In the Authentication section, select “Use these credentials”. The User name is your service provider login name (generally the part before @service.com,) the Password is the app password you obtained earlier, entered without spaces. Finally, Save & Close.
Where you find the app password generation for your provider will vary. In the case of Yahoo, it is the following:
I left the CardDAV tab in the default state, Use identity credentials, and it worked too. But that might not always work. You might need to set User name and Password there also.