The .mxl file is created, but when I try to open it using MuseScore it complains about the file not being valid. The command musicxml2ly also complains about the same.

Converts .MXL (MusicXML) files to the Swords of Legends Online .GJM file format Note: This is 100% a "work in progress" and is rather hacky and garbage code. It might get better over time.