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Unable to restore outlook.ost file size of about 15 GB consequence of using scanpst.exe and pst2gb.
Scanpst on the eighth stage of checking an error "The test was aborted because or error, the file was not changed"
Pst2gb at the opening of the file produces an error 52

You can apply next 5 steps or make use of https://www.openfiletool.com/ostopen.html OST Open File Tool

1. Open Microsoft Outlook. On the "Tools" menu, select "Email Accounts." Choose "View or Change Existing E-mail Accounts," and click "Next."
2. Select the Exchange Server, click "Change" and then click "More Settings." Click "Manually Control Connection State" so that Outlook will start offline the next time it is opened. Click "Work Offline and Use Dial-up Networking" so that it will not try to connect to the server. Close Outlook.
3. Open Outlook again. Select "Work Offline" at the prompt. Select "Services" from the "Tools" menu. The "Services" dialog box will open.
4. Click the "Add" button on the "Services" tab of the "Services" dialog box. Select "Personal Folders" and then click "OK."
5. Enter a name for the folder, and select the Desktop as the location. Click "OK" to select default settings. Drag the "Personal Folder" from the Outlook window to the new folder to move OST files to the personal folder. When the computer syncs with the server, all of the OST files can be copied there.