Thanks for your reply. I have 2 additional "accounts" already in DAVdroid added by direct URL:
I added them when DAVdroid on my phone and Zimbra on the server were at a older versions.
But when I add a new "account" today with URL /firstname.lastname@example.org/Calendar/ now, it shows me the result of the request to /email@example.com/. The URL is definitely correct (used by Thunderbird/Lightning). In the DAVdroid logs first thing I can see is the command PROPFIND https://firstname.lastname@example.org/ http/1.1. mthoben is the username I put in when adding a new account in DAVdroid.
Anything I can do to see more of the requests sent and responses received by DAVdroid? Or did I just enable log to external file too late?
Thanks for your help!