The set of files cannot be opened - Is not an Outlook data file

Question

Hi there!

I am the Exchange admin for my business and am running into a peculiar issue while upgrading from Exchange 2010 to Exchange 2016 in one of our environments.

For reference, we are using the latest rollup for ex2010, the latest cu (20) for ex2016, and up to date Outlook 2016 clients.

The issue is as follows:

I have installed Exchange 2016 on a fresh Server 2016 machine. My MX DNS record is still pointing at our Exchange 2010 (Server 2008) server and I have not migrated any mailboxes or anything over to Exchange 2016 - I've also updated the autodiscoverinternalserviceuri on the Exchange 2016 server to the correct value. So, all Exchange traffic is still routing through Autodiscover to the Exchange 2010 server correctly.

So - next item of business was to take a clone of our secondary domain controller, exchange 2010/2016 servers, and a workstation in our environment to do a test migration. We attempted to get our primary domain controller to play nice, but we couldn't get it working - in the end we used the DC2 clone to size FSMO roles and operate as the PDC. We completely cleaned out the Metadata and DNS records, etc from DC1 so I don't believe this part is the issue.

All the servers function normally and OWA works fine. The issue occurs when a user tries to open Outlook on the test workstation for the first time or from an already-created Outlook profile/ost file. We tested this with an already-created Windows profile and the same one completely from scratch (remove Windows local profile and all associated files) Outlook opens, correctly autodiscovers the user's details and fills out the various fields it needs to, and seems to start loading the program (blue ribbon appears that shows loading messages)

After a second or two, an error appears stating that "The set of files cannot be opened.../.../blahblahblah.ost is not an Outlook data file. After acknowledging the error message, Outlook will then close itself.

Now - I have tried almost everything under the sun to fix this issue...and I'm thinking it must be something to do with Autodiscover and the connection to Exchange itself. I've tried all google-able things, from the Outlook data file repair tool to registry changes to cached mode, to wiping Outlook/Windows profiles and all those .xmls and using /resetnavpane - you name it. The thing that leads me to believe that this is not a workstation or account issue is that the .OST file that Outlook generates is 0KB in size. How can the OST file be corrupt if there is nothing in it and this is a first time fresh Windows/Outlook profile? Also, Outlook 2016 installed on the Exchange server itself works completely fine...so there has to be something going on with the Autodiscover connection being messed with in some way on our network. We tried sniffing out the issue using WireShark but didn't get very far with that either.

My guess is that this error message (that seems to have quite a few common varieties as google hints at) is at times very specific and correct about an issue and sometimes very vague and misleading to what the issue actually is...

We have tried the Exchange upgrade in another completely fresh sandbox environment with brand new Domain Controllers, exchange 2010 and 2016 server installs and run into more or less the same issue with all of the internal URI's/SCP's updated and all the other ducks in a row Exchange-wise. The interesting thing is that on our live main environment that I described earlier (Exchange 2016 running but not handling any mailboxes or traffic yet) there are no issues creating a fresh Outlook profile on a fresh Windows profile at all. My worry is that once I start migrating mailboxes, this will start to occur.

Any suggestions/ideas are welcome! Thank you for your time.

Answer 1

Hi anonymous user Hopman,

Welcome to Microsoft Q&A!

May I understand that your situation like below:
In order to upgrade your current environment from Exchange 2010 to Exchange 2016, you did the migration in your test environment, install an Exchange 2010 and Exchange 2016 coexistence environment, migrate users from Exchange 2010 to 2016. However, those migrated users will encounter errors when configuring their email account to Outlook desktop client.

If this is your case,

so there has to be something going on with the Autodiscover connection being messed with in some way on our network.

In order to conduct further research about your issue, please try the suggestions below to troubleshot:

1. Does this error only happen when adding specific email account to Outlook desktop client? Or happen to all the users within the organization? It is suggested for you to do a few more tests using some other user mailbox to see if the issue has any difference.
2. In some similar cases, the Autodiscover Application Pool usually needs to be restarted after migrating mailbox. So, please try to restart and recycle the Autodiscover Application Pool on the Exchange Server 2016 via the command below and then try again to see if the issue would has any difference.
3. To test if Autodiscover is working, please try the “Test E-mail AutoConfiguration…” and see if there is any error in the test results.
   (Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
4. By the way, are there any errors or failures from your server side(Event errors for example)?

Any update, please feel free to post back.

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