The error Can't open this item. Unable to open the free/busy information can appear when accessing Outlook calendars.
Two possible fixes were found at this webpage. The first one has been proved to work; both are pasted here:
Article ID: 146
Created On: Wed, Jun 28, 2017 at 5:03 PM
Last Updated On: Wed, Jun 28, 2017 at 5:03 PM
Online URL: https://knowledgebase.stickman.co.uk/article.php?id=146