Before any repairs, try VLC Media Player. VLC has built-in error concealment and can often play partially corrupted MP4 files that Windows Media Player or QuickTime cannot.

Steps:

If this works, re-encode the file: Use VLC’s Convert/Save feature to generate a clean, working MP4 copy.

If the file plays audio but no video (or vice versa), the streams might be misaligned. FFmpeg can "rewrite" the file without re-encoding.

Command:

ffmpeg -i input_ss_julia.mp4 -c copy -map 0 output_fixed.mp4

This copies all streams directly into a new, clean MP4 container. It’s fast and lossless.

Less likely, but possible. Some video files come with a lightweight DRM (Digital Rights Management) lock. A "fix" in this context would mean removing that DRM to allow playback on any device.