Thanks for the update. To isolate the issue, please answer the questions below:
- What's the current build and version of your Windows computer? Please type in winver in Cortana to determine the current build and version of your Windows computer.
- Are only .mp4 files are affected? What about other file formats?
- Do you have full permissions on the file?
For now, let's create another user account and see if the issue will persist. Click on this link on how to Create a local user or administrator account in Windows 10.
Let us know how it goes.