https://davmail.msx.tu-dresden.de:1180/users/[zih-login_of_calendarowner@tu-dresden.de]/calendar/
Access to sub-calendars via Thunderbird is currently only possible via DavMail:
https://davmail.msx.tu-dresden.de:1180/users/[zih-login_of_calendarowner]/calendar/subcalendar/
Open the calendar view in Thunderbird and click on the + icon by the calendar list.
Select "On the network" and click on "Next".
Enter user\zih-login for username and the following address (make sure that you change your ZIH login in the address and do not simply copy the link).
https://davmail.msx.tu-dresden.de:1180/users/zih-login/Kalender
Then click on "Find calendars".
Now enter the password of your ZIH login and confirm this with "Sign in".
Now select the calendars you want to add and press "Subscribe".
You can now see your calendars in Thunderbird.