This error is caused by UpdatePanel which is only able to send a limited response to the browser during update.
There are several options:
1. You can place the DayPilotScheduler control outside of the UpdatePanel.
2. You can keep DayPilotScheduler inside the UpdatePanel and load events using a internal CallBack mechanism - the initial event set can be loaded using Init event, and all events allow "CallBack" handling type.
It also indicates that you data set is big and you should make sure that you are only loading the required data. The scheduler control filters out events that are outside of the visible range (VisibleStart, VisibleEnd) before sending them back to the client side. However, if the visible range is too long you may still hit the limit.
You can also consider using the dynamic event loading mode (https://doc.daypilot.org/scheduler/dynamic-event-loading/) that lets the Scheduler load events dynamically during scrolling.