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 turn off shared calendar improvements:
- In Outlook, choose File | Account Settings | Account Settings
- Double click on your account
- Click More Settings
- Click the Advanced tab.
- De-select "Turn on Shared Calendar Improvements" in the Microsoft 365 Features section.
- Click OK
- Close Outlook
- Restart Outlook
- Re-open the calendar again. NOTE: you may need to delete the calendar from your list of calendars that were previously opened.
Microsoft has said that turning off the feature doesn't cause you to miss out on any currently working features and your calendar will function as expected.