Here is an extract from our General Troubleshooting
Design-Time error on a page using RadControl(s):
Error Creating Control - RadControl
'Property' cannot be initialized. Details: Unable to cast object of type 'Type' to type 'Type'.
If the Telerik.Web.UI.dll assembly is updated while a page using RadControl(s) is in Design-Time, VisualStudio creates two different versions of the assembly, hence two different versions of the same RadControl(s) are available.
Restart Visual Studio.
I hope this helps.
the Telerik team
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