I tried your image for the rock pi 3a and followed all other instructions to get ffmpeg. I get the following when running this:
ffmpeg -i rtsp://... -f webm -r 20 -s 640x400 -vcodec vp8 clip1.webm
[h264_rkmpp @ 0xaaab1b1af850] Failed to initialize MPP context (code = -1).
[h264_rkmpp @ 0xaaab1b1af850] Failed to initialize RKMPP decoder.
Stream mapping:
Stream #0:0 -> #0:0 (h264 (h264_rkmpp) -> vp8 (libvpx))
Stream #0:1 -> #0:1 (pcm_alaw (native) -> opus (libopus))
Error while opening decoder for input stream #0:0 : Unknown error occurred
Do you have any idea where I am doing something wrong?