I am using eM client for my yahoo email account and eM Client emclient v9.0.1317
I get a Server Error unter Carddav and CalDAV (IMAP is still working fine):
StatusCode: 500, ReasonPhrase: ‘Server Error’, Version: 1.1, Content: System.Net.Http.HttpConnectionResponseContent, Headers:
The only way (workaround) for my opinion is, when you using Yahoo and eM Client, to generate and manage third-party app passwords
If you see the username and password fields in your account settings you might need to replace your main password with one specifically created for eM Client.
Please read more about App passwords (and how to create one) here - Generate and manage third-party app passwords | Yahoo Help - SLN15241
CalDav has with this workaround still sync problems from time to time, this is definitive a Yahoo Problem, but you can see your Calendar and your contacts again with the third-party app passwords.
I don’t now is this a design problem from eM Client or Yahoo, but if you using Yahoo the Secure sign-in method - oAuth is only still working for IMAP.
Maybe that helps eM Client User with a Yahoo Account.