.ost files become corrupted and need to be repaired or replaced sporadically
Frequently accessed Outlook files with .pst and .ost extensions can be subject to corruption if anti virus software does file-level scanning on them while outlook is in use.
Microsoft recommends not scanning these files on the Outlook client. See Planning considerations for deploying Outlook 2016 for Windows for more detail.
To prevent SEPC from scanning and potentially corrupting these files, you can setup a file extension exclusion: Setting up Exclusions in Endpoint Protection Cloud