Home page starts with https
After you connect to the network and launch a web browser, the web browser tries to display your default home page. This request is (usually) redirected to the Google WiFi login page.
If the default home page is a secure web page (that is, starting with https), the redirection does not work, and an error page is displayed. When this happens, simply enter a non-secure URL (such as www.google.com) and the browser redirects to the Google WiFi login page, as intended. If you don't want to have to do this extra typing, set your home page to any URL that starts with http.