Bubble error on production

this is the error that I'm getting :
Uncaught ReferenceError: s is not defined
at DayPilot.Bubble.show (app-0c134acd3f.js:26)
at app-0c134acd3f.js:28

What version of DayPilot do you use?

The following demo which uses version 8.4.2911 (the latest official release) seems to work fine:
https://javascript.daypilot.org/demo/scheduler/rowheaderactiveareas.html

Code sample:

dp.cellBubble = new DayPilot.Bubble({onLoad:function(args) {
  var cell = args.source;
  args.html = cell.start.toString("d MMMM yyyy");
}});

That's the version that I'm using: 2911

this is the function for showing the bubble -

dc.cellBubble = new DayPilot.Bubble({
onLoad: function (args) {
var cellStart = args.source.start.toString('d MMMM yyyy');
args.html = cellStart + " " + args.source.resource;
}
});

and again, the weird thing is that its work fine locally (I have 3 kinds of bubbles) but on production its break.
thinking about adding tooltip instead.