In Microsoft Outlook 2013/2016/2019/Microsoft365 application you run across the following popup at startup:
“Outlook cannot open your default email folders. Outlook is using an old copy of OST. Exit Outlook, delete the .ost file and restart Outlook. A new file will be automatically created the next time you initiate a send/receive”.
First step is to locate the problematic .ost file(s) and cut them from the original location to a backup location. The default location of the .ost file (unless it is changed by GPO) is C:\Users\[username]\AppData\Local\Microsoft\Outlook. After moving them to a backup location, then start Outlook again and the Outlook application should automatically recreate the .ost file. If this issue is not resolved, then proceed to the second step.
Second step is to run the scanpst.exe Microsoft tool, which can repair any corruptions present in the .ost file(s). There are alternative third party tools to Microsoft scanpst.exe, such as for instance the Stellar Repair Tool (https://www.stellarinfo.com/email-repair/outlook-pst-repair/buy-now.php). If this does not resolve the issue either, proceed to the next step.
Third step is to either use a third party tool for converting the .ost into .pst and try to salvage the data or if the data is stored in a cloud service and there is no risk of losing it, then your final resolution step is to create a new Microsoft Outlook profile and setup all user accounts under that new profile from the beginning.
Last but not least, if Microsoft Outlook is published from a hosted environment, then ensure that you take account of all other dependencies of that environment (for instance Citrix Profile Management and FsLogix profile management related issues).