There is a bug where the Kentico code will add a ScriptManager to the page when in Design mode (not at initial page load, just after any action like clicking on "move", "up", "down", etc.) even when a ScriptManager may already exist (often placed in the Master page by the web developer).

This creates an unhandled exception because only one ScriptManager can be on a page at a time. However, it has to be placed on the page by the web developer because the Design mode is the only place that Kentico puts it on its own. So if the developer doesn't put one on it won't work in every other place, especially the actual public live view.

The Kentico code needs to check for a ScriptManager existance before it manually adds it's own. (Sorry, I couldn't find the exact piece of code to submit a patch, since it is in the compiled DLLs of which my company hasn't purchased the source.)

See the following thread in the Kentico-Portal forum for more details and discussion:

http://www.kentico.com/DevNet/Forums.aspx?ForumID=16&Thread=00003879