ffmpeg overlay tranparency video loop
If you're using a recent git build, use stream_loop and drop the loop filter:

-stream_loop -1 -c:v libvpx-vp9 -i Overlay.webm -filter_complex "[0][1]overlay=...


For older builds, you'll have to supply the loop filter with full details,

[1]loop=-1:C:0,setpts=N/FRAME_RATE/TB[bg]


where C is a value greater than the frame count of the file.