Try removing the control you have added from the problematic page or just comment the web.config line inside toolboxControls used for registering the toolbox item.
ScriptResourceHandler - handler for processing requests for script files that are embedded as resources in an assembly. Try deleting or comment out all the system.web.extensions
configuration section definitions and configuration section group definitions from the web.config file. Make sure that the website is not build against .NET Framework 4.0 and the application pool is running under.NET Framework 3.5 or 2.0
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