There is no way how you could ever convert .xspf playlists to MP4, or any other audio format.
Playlists are just special text files that contain sequence of the actual .mp4 (or other audio) files to be played. At best, you can check the .xspf file using a text editor and see where the actual audio files are located, but usually these are located within the same folder.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.