I am getting constant [CardDAV] errors of the form :
[CardDAV] Sync of ‘email address’/Contacts/failed due to the following error : ‘email address’ (Server Hangup)
I have a Sky and a Yahoo account and both show these errors. What is the problem?
Doesn’t Sky use Yahoo, so they are essentially the same server?
Yahoo has been having some problems recently, as you would expect from a multi-billion dollar company that obviously doesn’t have the resources to hire competent technicians. 
Your best bet is to contact their support if it was working in eM Client and now it’s not.