I just noticed that my calendar is only visible to anonymous/unauthenticated users and to Administrators.
When someone logs in as a subscriber (anything other than administrator), the calendar will not load. It's just a blank screen.
For a long time I didn't notice because I had no members who logged in. Only me (the admin).
Hopefully someone can help me to resolve this.
NOTE: I am using v1.2.3 of the Stout Google Calendar plugin.