I've seen a similar error that was caused by an interference with Gaia framework.
Basically, it was caused by an incompatible JSON serialization used in Object.prototype.toJSON added to the global space by Gaia. What they are doing is different from the JSON serialization "standard" implementation available at json.org (their toJSON method produces "[...]" instead of plain [...]).
What they are doing with the prototype object is not correct, I believe. Anyway, I have added a fix for this behavior in DayPilot build 1941.
Please let me know if you experience this issue with the currect build as well.