We have seen an increase in issues of users having trouble opening shared calendars and resources and noticed that the users do have permissions to the mailboxes. 

A solution that Microsoft provided was to temporarily turn off shared calendar improvements:

1. In Outlook, choose File | Account Settings | Account Settings
2. Double click on your account
3. Click More Settings
4. Click the Advanced tab.
5. De-select "Turn on Shared Calendar Improvements" in the Microsoft 365 Features section.
6. Click OK
7. Close Outlook
8. Restart Outlook
9. Re-open the calendar again. NOTE:  you may need to delete the calendar from your list of calendars that were previously opened.