OST files gone and profile directory changed after Windows 10 upgrade

That has happened to a number of people. :(  The ost is probably under the windows.old folder.   Right click on your account name in the folder list and choose Open File Location. This is where the ost is. 

Try repairing the account - Go to File, Account Settings, open the account settings dialog. Select the account and click Repair.  If that doesn't fix it, remove the account and add it back. If you use IMAP and have calendar, contacts, or tasks in folders named 'this computer only', first export the folders to a CSV or pst. 

On the colors:  It sounds like you are syncing settings. Start typing  Sync your settings  on the Start menu and when it comes up, open it. Turn off the sync options. 

OK for some reason I can now find the ost (and pst files for the two mailboxes I have defined in outlook). I have no idea why I have a new profile directory after the upgrade username.000 but anyway...

After more investigation it appears that the error is only with 1 of the two mailboxes. As the other one can send and receive files.

The send receive error states 

"Sending reported error (0x8004010F): 'Outlook Data file cannot be accessed"

But the name and location of the data files defined in the mail setup are correct

I looked at the data files and I could only see one thing

The "Opens with" value on the file properties (for all the ost and pst files) States "Unknown Application". Even though the type column in file explorer says "Outlook data file" and the File Icon has the Outlook logo

I tried to specify the full path for outlook.exe by clicking on the "Opens with button"

C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXEbut all that happens is Outlook opens and the "Opens with" value does not change. Also the file explorer dialogue box opens again as if I hadn't set out as the "opens with" value?

But I don't think that is the problem as the other ost for the other mailbox is set the same way

I also went to default apps and set the default mail app to Outlook 

I have attached two screen shots of the file in question (not the security settings are identical on the mailbox ost that is working

I have also attached a screenshot of the error when I click on the file in the mail settings dialogue from control panel (I don't get this error when I click on the other files).

As its an ost file I deleted the file and created a new ost by resyncing the content back from the server again.

I still get the same error as soon as I try to send an email from that mailbox. It can receive messages but can't send!? As soon as I hit send it reports the same error 

"Sending reported error (0x8004010F): 'Outlook Data file cannot be accessed"

The message sits in the outbox of the functioning mailbox (I assume it can't sit in the outbox of the mailbox that doesn't work due to the file issue).

So I completely deleted the mailbox (and the ost) and set it up from scratch and it appears to be working (emails being sent out).

No idea what happened and cant help assume it was a present from the MS Windows 10 upgrade team (thanks folks, that's a few hours of my life I am not getting back!

Anyway thanks for your help. I hope this helps someone with similar problems

I'm helping someone with this issue. He lost all of the info in his calendar and contacts. Apparently that doesn't sync with his comcast.net account. Any recommendations on how to recover that data from the original file? 

Do you have a copy of the original ost file that contained the calendar and contacts? If yes, they can be recovered. If the ost was deleted, there is no way to recover it, unless you can get it back using disk recovery software. 

what you need to know about IMAP accounts in Outlook

I do have the original file. However, I can't add it Outlook. Windows says its not an Outlook Data File. 

An ost can only be opened by the profile that created it.  

Data files cannot be opened by double clicking on them (ost or pst).

Is the current profile the one that created it?  If not, when was this profile created?  (You might be able to get the profile back using system restore) 

How big is the ost? 

If you no longer have the profile, you'll need 3rd party ost to pst software to export it.

https://www.slipstick.com/problems/pst-repair/convert-an-exchange-offline-file-ost-to-a-personal-file-pst/