For those who use Apple Calendar Server (iCal server), you must manually enter the URL of your account

URL should be in the following form:

http://FQDN:8008/calendars/users/USER...

Replace 'USERNAME' with your username. Notice the slash "/" that ends the URL is crucial.

The last "calendar/" isn't necessary - but depending on how you would like to see the calendar in eM Client, it points to the calendar root directory, you may leave it there.