Hi,
We had this problem for a while.
We were using a master page and loading in ascx files containing the daypilot controls. We were also using ajax and update panels.
What we found was that to reproduce we needed to be running a system where it took longer to download the HTML than to execute the scripts on the page.
The cause of the problem for us wasthe DayPilot control in the UIMaster page (which had to be there to prevent a daypilot control not found error). This tried to modify the DOM as soon as itwas placed on the page, which may or may not be after the page has finished downloading.
This wasfixed by putting the daypilot control in the master page in a non-visible div that is run at the server.
This has worked for us and we haven't had the problem reoccur at all. Hopefully there might be something on this that can help you.
Darren