I get a popup with a very generic error (this is from browser - web):
The message is in italian it means more or less: "It was not possible to process your multimedia file".
Another little piece of information, since I did a few more tries.
An mp4 generated in a similar way (so again with ffmpeg muxing a still image and audio) worked fine.
Here's the GSPOT report from the WORKING mp4:
There are a few differences, i.e. the frame rate (which is lower) and bit rate of video (also lower).
AND (IMHO most important) the aspect ratio is different.