Hi Hui,
Thank you for contacting Syncfusion support.
Could please answer the following queries, it will help us understand your issue and provide solution.
1.What controls are your using in your page? Are you using waiting popup control in your sample?
2.Have you referenced the waiting popup.gif file in your sample anywhere?
3.Can you please provide an issue reproducible sample or code snippet to reproduce the issue at our end?
Regards,
Prince