Broken after 4.1 upgrade [custom widgets]
After my 4.1 upgrade I too have the content placeholder issue described here:
Unfortunatly another error has occured as well:
When I run the site in VS2010 and attempt to modify a page or template by dragging an item onto the page, a dialog box pops up and says: message from webpage: undefined
Running from a published version, i'm just not able to grab any widgets, controls or layout areas.
I have tried to reproduce your issue in VS but didn't got the same result.
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the Telerik team