I follow now. I apologize, I did not even think there would be a second calendar so I did not bother logging in.
So I am seeing that issue now too. Our plugin is completely compatible with WP 3.8, so that is not the real problem. This looks like a conflict with your theme or plugin, and the new version. One of these things is likely overriding a query when it should not.
If you are interested in diving in and trying to find out what the conflict is, I would first try disabling your theme. If that does not fix it, your plugins. Eventually disabling one of these should temporarily fix the problem. Once you have isolated which of those is causing a conflict we can dive into this a bit further.
Please let me know if that helps you to isolate the issue. I would be glad to help you further if I can. Thanks!
– Brook