Office 365 Shared mailboxes in Outlook

We have recently migrated Exchange from 2010 on premise to O365 in the cloud. We are able to access shared mailboxes through owa but the shared mailbox does not show in Outlook on the desktop. I can add the mailbox manually but it takes a minute or two for the process to complete and then when I try to open the shared mailbox I get this error..."Cannot expand the folder. The set of folders cannot be opened. The file D"\users\%username%\appdata\local\Microsoft\%email address% is in use and cannot be accessed. Close any application that is using this file, and then try again. You might need to restart your computer."

Any help is appreciated.
Jerry KassilSystem EngineerAsked:
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Vasil Michev (MVP)Commented:
First of all, where are the mailboxes located? There are some implications regarding cross-premises Full Access permissions, such as no support for Automapping, and even more implications for folder-level permissions.
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Jerry KassilSystem EngineerAuthor Commented:
All mailboxes were migrated to the cloud. They were then converted to shared mailboxes and permissions given to targeted user accounts.
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Jerry KassilSystem EngineerAuthor Commented:
Accessing the shared mailbox through owa works without issue.
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Michael B. SmithExchange & Active Directory ExpertCommented:
Have you tried the most recent version of Outlook?
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Jerry KassilSystem EngineerAuthor Commented:
Yes, we are running Outlook 2016
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Michael B. SmithExchange & Active Directory ExpertCommented:
exactly how are you adding the mailbox?
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Jerry KassilSystem EngineerAuthor Commented:
My understanding is that is should autodiscover and just show up. Given that is not happening, File>Account Settings, Change Account, More Settings, Advanced tab, add the mailbox.
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Vasil Michev (MVP)Commented:
Automapping relies on properly working autodiscover, so you might as well check for that. Hold the CTRL key, right-click on the Outlook icon in the tray, select Test E-mail autoconfiguration, enter UPN and password, clear the two Guessmart checkoboxes and run the test. Once you obtain the XML file, scroll down to the bottom and look for the <AlternativeMailbox> element. If it's missing, you will have to re-add the Full Access permissions.

One more question - do you see all folders from the shared mailbox in OWA? It might be that you simply have folder-level access, in which case Outlook will fail to dispay the folder(s) unless you have at least FolderVisible permissions on any "parent" folder in the path, the Root/Top of information store one included.
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Jerry KassilSystem EngineerAuthor Commented:
Yes, AlternativeMailbox entries are there. From owa, all folders are visible and working as expected.
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Saif ShaikhServer engineer Commented:
Try to provide full access permission to the users on the shared mailbox from o365 powershell or using global admin credentials expand the exchange tab and edit the shared mailbox and click on delegate access tab and the click on full access and then add the user and see if shared mailbox comes up in the outlook.
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Jerry KassilSystem EngineerAuthor Commented:
I have been through those steps with no success. What's curious is that as of about an hour ago, it is now showing up. The one thing i did do was to enable Outlook desktop (MAPI) form the shared mailboxes Email Apps section. Do you think this mailbox needs an E3 license or is being a Shared mailbox enough?
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Vasil Michev (MVP)Commented:
There's no need for a license unless the shared mailbox is over 50Gb in size, or you need advanced functionality such as Online Archive or Hold. MAPI should be enabled on the mailbox though, I'm not sure why you had it disabled in the first place.
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Jerry KassilSystem EngineerAuthor Commented:
Newbie...
I removed the F1 license and then went into the shared mailbox settings to enable MAPI and i get
..."License validation error: the action 'Set-CASMailbox', 'MAPIEnabled', can't be performed on the user '%username% with license 'BPOS_S_Deskless'.
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Jerry KassilSystem EngineerAuthor Commented:
So for Grins and Giggles and I hit Save again and this time it accepted the change...latency?
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Vasil Michev (MVP)Commented:
Kiosk (F1) users cannot use Outlook (have the MAPI protocol disabled), so that's why probably. Again, you dont need any license assigned to the shared mailbox, unless over 50GB or you need hold/archive.
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