I got a lot these errors when adding tracks/playlists:
POST https://api.music.apple.com/v1/me/library/playlists
{"errors":[{"id":"INX45ZUMMU3X5IOJXDFSYKMFY4","title":"Internal Service Error","status":"500","code":"50000"}]}
POST https://api.music.apple.com/v1/me/library/playlists
{"errors":[{"id":"X6N5JWHWOPS2XH6Y35AACK4MOU","title":"Internal Service Error","status":"500","code":"50000"}]}
POST https://api.music.apple.com/v1/me/library/playlists
{"errors":[{"id":"EHS25JSUVBQR4Q5MALSZJLQM4Y","title":"Internal Service Error","status":"500","code":"50000"}]}
POST https://api.music.apple.com/v1/me/library/playlists
{"errors":[{"id":"KV4SPLAGH3U7L2O5LAJDXFD3P4","title":"Internal Service Error","status":"500","code":"50000"}]}
POST https://api.music.apple.com/v1/me/library/playlists
{"errors":[{"id":"FOM7LPKXJXXZVNMCF44CCEKTRE","title":"Internal Service Error","status":"500","code":"50000"}]}