If you find that you cannot open calendars, please try the following fix:
1) In Outlook, go to File > Account Settings > Account Settings.
2) Double click your mail profile and go to More Settings.
3) Go to the Advanced tab and untick "Turn on shared calendar improvements" under Microsoft 365 features and click Apply or OK.
4) You will be prompted to restart Outlook to apply the changes. Confirm this and close all Outlook windows ensuring any draft emails are saved.
5) Re-open Outlook and try to add the relevant calendar again, it should work without any issues.
6) If it doesn't work raise a helpdesk ticket or respond to the open one you received these instructions on, as further changes may be required.