Sry no. I don´t use Windows.
I convert under Linux with ffmpeg and i have tried today h.264
libx264 libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (codec h264)
and dont work
but i see ffmpeg have a second typ like
libx264rgb libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 RGB (codec h264)
dont know the difference but i test it.
that mpeg4 are not working anymore is bad.
the videos with your converter works on older firmware 16.x too ???
your converter use ffmpeg too what kind of options you use ?
I convert under Linux with ffmpeg and i have tried today h.264
libx264 libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (codec h264)
and dont work

but i see ffmpeg have a second typ like
libx264rgb libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 RGB (codec h264)
dont know the difference but i test it.
that mpeg4 are not working anymore is bad.
the videos with your converter works on older firmware 16.x too ???
your converter use ffmpeg too what kind of options you use ?