This happens because the "args" object has been extended with new properties (including a reference to DayPilot.Scheduler instance) and it can't be serialized using JSON.stringify() anymore.
It's necessary to replace the "args" object in DayPilot.request calls with a custom object that selects the required properties, e.g.
dp.onEventMoved = function (args) {
var params = { id: args.e.id(), newStart: args.newStart, newEnd: args.newEnd, newResource: args.newResource };
DayPilot.request(
"backend_move.php",
function(req) { // success
var response = eval("(" + req.responseText + ")");
if (response && response.result) {
dp.message("Moved: " + response.message);
}
},
params, // used to be args
function(req) { // error
dp.message("Saving failed");
}
);
};