If automatic subscription does not work, you can subscribe to a calendar manually.
On the Kalenders page, select a calendar you like.
Click Selecteren.
If you are not logged in to WebCal.Guru, you will not be able to modify subscription settings and you will have significantly smaller selection of calendars. In this case, jump directly to Step 4.
Adjust the subscription settings if needed and click Aan Mijn kalenders toevoegen.
You will be taken to Mijn kalenders page. On the calendar you just added, click next to Downloaden and select Adres naar klembord kopiëren.
Open the Edit menu and select New Calendar Subscription.
Click the Calendar URL field and paste the calendar address from the clipboard.
Click Subscribe.
Usually the default settings are fine. Just make sure Auto-refresh is set, for example Every day or Every week. Usually the Location should be iCloud, but On My Mac also works fine. Click OK.
Updated 8 Feb 2023