As you have already added your mail account previously without the Calendar, you can then just manually add a “CalDAV” Web.de calendar via URL as per @Gary post below on the following thread.
Go to “Menu / Accounts / Add Account / Calendar / CalDAV” (Pc) or “eM Client / Accounts / Add Account / Calendar / CalDAV” (Mac) and following the prompts.
https://forum.emclient.com/t/gmx-web-de-caldav-probleme/49604/2
@Gary post extract below from the above thread.
