When loading an event page from your calendar, you recieve this 403 Error:

Something went wrong while fetching events.
The request status is: 403
The error thrown was: Forbidden

Do you have a plugin installed called iThemes Security Pro?

It’s could be a setting in that plugin. The one that triggered it for other users was: Suspicious Query Strings – Filter Suspicious Query Strings in the URL.¬†¬†Unchecking this option fixed the problem.