So, there's this thread:

https://forum.kde.org/viewtopic.php?f=281&t=152896&p=400602#p400602

The problem for that person is that somehow Krita is not exporting ANY
frames, no matter what size, what amount, it just bugs out.

I am too busy to do programming right now due school, and I don't
understand the animation rendering code at all, so can someone who
does understand it perhaps add better debug than 'krita couldn't
render the frames'?

It is NOT because of ffmpeg, and I suspect a lot more people have
problems that are unrelated to ffmpeg, but we cannot debug them
because the error messages are so terrible.

Another thing, as noted in the thread, windows media player, the
wholely incompetent program it is, cannot play mp4 files that don't
use the 'baseline' profile. I already set the default to pick up
'baseline' instead for 4.0, but we still get people have trouble with
that. Is it possible to force mp4 on Windows to use the baseline
profile regardless?

Finally, note how the person for that last thing says that there is an
error, but only later says they already could play it back. Apparantly
they don't understand that if there's an error with playback, they
maybe should say that the video player gives the error, not Krita.
Keep this in mind when people say there's errors with animation
export.

-- 
Wolthera