After restarting my system during work on ASP.NET i found popup indicating 'UpdatePanel could not be set on property ContentTemplate' on design view of same pages which were running smoothly before system boot.
After some googling and other searches here's how i fixed my problem:
1. Close all the pages in visual studio and exit VS environment entirely.
2. Open C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files and delete your [Application Name] folder.
3. Restart VS
I did not require to restart IIS, but it can be a useful step for this fix too. This problem may occur in both VS 2005 and VS2008.
