You probably need to change what directory you're saving to -- don't save to a folder that has specific user permissions (like anything in your user folder has, including your desktop).
Also, don't record to mp4. Ever. If anything happens during the recording and it can't write the closing data, then the entire file will be unplayable, with no way to recover.
Use .mkv. If you absolutely need to use mp4, then save to mkv first then use any number of utilities out there to wrap it into an mp4 container (you can do this without re-encoding even).
