Error playing video from snapchat (.mp4)
Seems that Snapchat allows to select multiple clips of different parameters and stitch them together as one MP4.
Like you said, it plays fine in VLC, but ffplay here warns that

[mov,mp4,m4a,3gp,3g2,mj2 @ 00000000005b0a00] Concatenated H.264 or
H.265 might not play correctly.
[mov,mp4,m4a,3gp,3g2,mj2 @&gt; 00000000005ba100] ignoring multiple glbl
[mov,mp4,m4a,3gp,3g2,mj2 @&gt; 00000000005b0a00] multiple edit list entries, a/v desync might occur, patch welcome

VLC fails to convert to a good file. Crude method would to screen-record the video playback and crop that, or Snapchat seems to allow to download the individual clips, which would be the best possible method for now.