I downloaded a new copy of FFmpeg from here https://www.ffmpeg.org/download.html#build-windows

AND

Maybe more importantly I just waited for the cmd windows to close by themselves. I think the slower your PC and the bigger the video the longer they will stay open. Just have some patience and let FFmpeg run and all good.

I am having the same problem as the OP. I get two cmd windows that appear to have text in them, however the windows are black background with black text which does not appear if you you try to highlight. I have tried to change the BG and Text colours with no effect.

I have managed to produce at least one video recently as an MP4 with FFmpeg.

Maybe I will try to download FFmpeg from somewhere else, but any advice on this issue gratefully received.