...
Re-add the mailbox using the recommended method for adding Shared Mailboxes as outlined below, or alternatively you can use the UIS https://help.uis.cam.ac.uk/service/collaboration/365/shared-mailboxes guides if you prefer
Windows device - https://cscs-itsupport.atlassian.net/l/cp/4KbSvBuL
Mac device - https://cscs-itsupport.atlassian.net/l/cp/N0btTMH6
Error: Something went wrong - The maximum number of Microsoft Exchange accounts has been added
...
There is a limit of 10 accounts that can be added to Outlook by default. Contact the CSCS Service Desk to have that number increased.