Public shared calendar refresh every 7 seconds

Our public shared calendar reloads the whole page every 7 seconds. Why?

Steps to reproduce:
1: sign in in any account
2: create a new calendar in the calendar app
3: share this calendar public
4: copy the link into clipboard
5: logout from owncloud as any anonymous public user
6: paste the link into a new tab

debian Linux 9.4
apache 2.4
owncloud server community edition

The Calendar App Version 1.5.8-rc1 works!