Can't subscribe to shared calendars

hi nextcloud,

i have set up some calendars in my nextcloud which i am succefully syncing with different client.
when i try to subscribe to shared calendars (in my nc or other users - also on other nc-servers) i can’t.

i am doing the following:

official calendar app
in one of the calendars (all affected) click on the share-symbol, check “share link”
click on … and on “link”
copy&paste link into on macos or busycal (my favorite calendar client) und calendar subscription.
both apps give me error messages: “Connection to failed”

What am I missing?
What is wrong?

Any help would be greatly appreciated.