Howdy Christian,
Sorry for the unusually long response time. We have had a much higher than normal number of new customers, causing a delay at times over our typical 24 hours response times.
Within Google Calendar you will be given two URLs, a private and a public one:

Is it possible you are using the private address? In order for anything to see that it will have to be logged in with your Google username and password, which WordPress does not have. So you need to share the public URL. When you click on the iCal button there it will take you to your public URL. It might prompt you to modify your privacy settings first.
Does that all make sense? Will that work for you? Please let me know.
Cheers!
– Brook