The error message you are encountering when trying to add a shared folder in New Outlook may be related to permission issues or connectivity problems. Since you mentioned that the user has full permissions and can access the inbox via classic Outlook, it suggests that the permissions are correctly set. However, there are a few things you can check:
- Permissions Verification: Double-check that the user has the correct permissions set for the shared mailbox in both the Exchange admin center and the Outlook settings.
- Hybrid Environment Issues: If you are in a hybrid environment, there could be complications with accessing shared mailboxes. Ensure that the mailbox permissions are correctly configured for hybrid scenarios, as issues can arise when trying to access mailboxes across different environments.
- Outlook Profile: Sometimes, issues can stem from the Outlook profile itself. Consider creating a new Outlook profile for the user to see if that resolves the issue.
- Network Connectivity: Ensure that there are no network issues affecting the connection to the mailbox. Sometimes, temporary connectivity issues can cause errors when accessing shared folders.
- Workaround: As a temporary solution, you might want to use the classic Outlook for accessing the mailbox until the issue with New Outlook is resolved.
If the problem persists, you may want to reach out to Microsoft support for further assistance.
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