@sunatomo 様
連投すみません。
Nextボタン見つかりました（笑）
操作時のログを貼っておきます。
このページは非標準の “zoom” プロパティを使用しています。関連するプロパティの値で calc() を使用するか、"transform" に加えて "transform-origin: 0 0" を使用することを検討してください。 smpd.local
connected mpd.js:24:189
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
function(a){return p.access(this,function(a){return a===m?p.text(this):this.empty().append((this[0]&&
this[0].ownerDocument||L).createTextNode(a))},null,a,arguments.length)}:click! mpd.js:54:66
app.setLocation:#/500 mpd.js:54:323
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
function(a){return p.access(this,function(a){return a===m?p.text(this):this.empty().append((this[0]&&
this[0].ownerDocument||L).createTextNode(a))},null,a,arguments.length)}:click! mpd.js:54:66
app.setLocation:#/0 mpd.js:54:323
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
function(a){return p.access(this,function(a){return a===m?p.text(this):this.empty().append((this[0]&&
this[0].ownerDocument||L).createTextNode(a))},null,a,arguments.length)}:click! mpd.js:54:66
app.setLocation:#/500 mpd.js:54:323
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
function(a){return p.access(this,function(a){return a===m?p.text(this):this.empty().append((this[0]&&
this[0].ownerDocument||L).createTextNode(a))},null,a,arguments.length)}:click! mpd.js:54:66
app.setLocation:#/1000 mpd.js:54:323
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
function(a){return p.access(this,function(a){return a===m?p.text(this):this.empty().append((this[0]&&
this[0].ownerDocument||L).createTextNode(a))},null,a,arguments.length)}:click! mpd.js:54:66
app.setLocation:#/500 mpd.js:54:323
execQueue #next show check1 mpd.js:30:89
execQueue #next.show check2 mpd.js:30:150
Syntaxエラーは出ていないようです。
このログはFirefox 74.0（64bit）です。
追記：
Nextボタンが出る状態で、曲送りをすると稀にSyntaxエラーがでるときがあるようです。20回に1回ぐらい？
Failed to load resource: the server responded with a status of 404 (Not Found)
/#/1500:1 Uncaught SyntaxError: Unexpected end of JSON input
at JSON.parse (<anonymous>)
at WebSocket.socket.onmessage (mpd.js:24)
socket.onmessage @ mpd.js:24
Charlie%20Haden%2FPat%20Metheny%20-%20Beyond%20The%20Missouri%20Sky%20(Short%20Stories).jpg:1 Failed to load resource: the server responded with a status of 404 (Not Found)
VM108:1 Uncaught SyntaxError: Unexpected end of JSON input
at JSON.parse (<anonymous>)
at WebSocket.socket.onmessage (mpd.js:24)
socket.onmessage @ mpd.js:24
Charlie%20Haden%2FPat%20Metheny%20-%20Beyond%20The%20Missouri%20Sky%20(Short%20Stories).jpg:1 GET http://smpd.local/NAS/CD_Wave/Charlie%20Haden/Beyond%20The%20Missouri%20Sky%20(Short%20Stories)/Charlie%20Haden%2FPat%20Metheny%20-%20Beyond%20The%20Missouri%20Sky%20(Short%20Stories).jpg 404 (Not Found)
