Subscribe iCal - Error message

I use Nextcloud 17.0.1 and I like to subscribe to an iCal calendar.;P;G&Tag=1&Zeit=9

But I get a error message:
Error requesting resource on remote server. This could possibly be related to a certificate mismatch.

I don’t know what is wrong with my installation.

Make sure that required intermediate and root certificates have been installed on your server:


OK, but how do I install these certificates at my Nextcloud