Hello,
This could happen if a user is not set as the owner of their own mailbox in Exchange. Such a situation could occur, for example, after a mailbox migration from an older version of Exchange. The issue can be fixed by running the following PowerShell script in the Exchange Management Shell:
Add-MailboxPermission "<user_DN>" -Owner "nt authority\self"
where <user_DN> is the distinguished name (DN) of the user you are having issues with (e.g. CN=John Doe,OU=New York Office,DC=example,DC=com).