Hi Rémi,
Sorry about the inconvenience caused.
We have analysed the reported issue with our sample and we are unable to reproduce the issue. Can you please revert us by modifying the sample based on your application along with the replication procedure? This would be helpful for us to provide better solution. We have prepared a sample for your reference. Please find the sample in the below location.
Sample Location: http://www.syncfusion.com/downloads/support/forum/119708/ze/GanttUpdatePanel-1826944549
Also can you please share more information regarding the actions done in the application which triggers the asynchronous PostBack.
Please let us know if you require further assistance on this.
Regards,
John R