FFmpeg concat problem
concat requires videos to have the same sample aspect ratio as well as the same resolution.

So, in this case,

ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref[wm][base];[wm]setsar=1[wm];[base][wm]concat=2" output.mp4


or

ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref[wm][base];[base][wm]concat=2:unsafe=1" output.mp4