This message indicates a source file that uses DivX-style packed B-frames. Packed bitstream isn't standard MPEG-4 and was used as a workaround for issues with the Video for Windows platform.
Unaddressed, there may be potential accuracy issues with fast seek and audio-visual syncing.
Execute the following with the source file as the input. Use the output as the source for the encode.
ffmpeg -i "input.avi" -codec copy -bsf:v "mpeg4_unpack_bframes" output.avi