While I can't figure out why animated png fails ...

Also, alias PIX does not work.

I used ' thirdparty/ffmpeg-5.1/ffmpeg -encoders'  output to see if there any uncovered codec, so I added few uncompressed video packers too ...

May be it need range of frames and not just one? (I used single-frame rendering)

Put those in ffmpeg/video folder (for source tree based single user build it will be in bin subfolder, but you can just put them in cinelerra-5.1/ffmpeg folder and they will be installed hopefully ...)