Hello @craig at Smash Balloon,
I have the same problem, and got the problem resolved.
I did as you said, verify if the plugin is active, insert the feed in a text widget (it works), but still got the “Invalid shortcode” error message.
By changing the title of the feed (“Appearance” -> “Widgets”) from Instagram Feed (I think Instagram was the default title) into Instagram, it works.
Regards,
