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:
- 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.