Office 365 (Outlook on the web) - Error - This calendar isn't available Please try again

When a user tries to access the Calendar by using Outlook on the web, that user receives the following error message:

This calendar isn't available. Please try again later.

Additionally, other users may be unable to see that user's free/busy information.

This problem can occur for one or more of the following reasons:

To resolve this problem, follow the steps outlined within Microsoft documentation.