I just learned that a new version of DivX (version 6) is available when I found a file on BitTorrent that mentioned DivX6 in the filename. Fortunately, the file uses the fourcc DX50 which implies that the bitstream syntax is the same as the previous version. Version 6 of the encoder likely features general improvements to the encoding engine but packs the data into the same bitstream (which is apparently compatible with ISO MPEG-4). The DX50 data decodes fine with FFmpeg.