Web.de address book shall be integrated via carddav. Web.de calendar integration via caldav works fine, but address book integration does not work. No error message, web.de address book is just not available. The server address is https://carddav.web.de as indicate in web.de’s help, username and paasword are the web.de username and password as with the calendar integration. Interestingly, the emClient contacts account shows caldav/carddav as protocol, whereas the calendar account shows caldav, only.
Solved: When the server address is https://carddav.web.de/CardDavProxy/carddav/current-user-principal-uri/, then the web.de contacts are synchronized with emClient