Accodring to the XSPF sepcification (v1), the identifier of a track MUST be a legal URI. xspf_playlist_node_build_file_item from xspf_playlist_node.module use nodes IDs as identifier. But an integer does not qualify as a legal URI and thus the generated XSPF are not valid.

Also, the specification is not clear, but it seems that the identifier URI must no resolve to the actual resource, so it should not be the URL of the file.

Since an identifier is not required for a track, the simplest fix is the remove it.