ffmpeg : error when added scale
Use

for %%I in ("*.mkv") do ffmpeg.exe -i "%%I" -i a.png -filter_complex "[0:v][1:v] overlay=25:25:enable='between(t,0,20)',scale=840:480[v]" -map [v] -map 0:a -codec:v libx264 -preset veryfast -crf 18 -codec:a copy "480\%%~nI.mkv"


All filters acting upon a stream should be within the same filtergraph.