Unable to add anything to the timeline

[PoisonedSlice]

Or the files being dragged are possibly in protected folders. That issue can be solved by reading this post.

Export fails with "[consumer avformat] Could not open"

Thank you for replying with that excellent workaround! https://support.microsoft.com/en-us/help/4046851/windows-10-allow-blocked-app-windows-security And from Protect important folders with controlled folder access on microsoft.com: Protected folders are specified when controlled folder access is configured. Typically, commonly used folders, such as those used for documents, pictures, downloads, and so on, are included in the list of controlled folders.