The file is not broken. It is illegal on that decoder.

A decoder advertises a level, and a level is three separate limits: how many macroblocks are in a frame, how many arrive per second, and how many bits per second. So the same pixels can be legal at one frame rate and refused at another — 1080p30 needs level 4.0 and 1080p60 needs 4.2; 4K30 needs 5.1 and 4K60 needs 5.2. A device one level short plays the first and shows a black screen with working audio for the second.

The engine is free and needs no account. A report that a video “does not work” never names the ceiling it hit, and there are more of them than the level. A bitrate is not a quality setting but bits per pixel, and the frame size already spent them: 10,000 kbit/s is 0.1608 at 1080p30 and 0.0201 at 2160p60, eight times less for the same number. A keyframe larger than the decoder's buffer stalls playback however fast the link is — once per keyframe, on a file a laptop plays perfectly. And an MP4's index is written last, so unless it is moved to the front nothing appears until the final byte lands: 67.5 MB over a 20 Mbit/s link is 27 seconds of nothing, invisible on the local copy where it was tested. Every lane ships a worked example that costs nothing. Writing the report needs credits.