Configuration errors generally display in the right (workspace) pane, when you first open the ControlPoint application or when you attempt to run a ControlPoint operation.
If you attempt to open ControlPoint and a blank page displays (with or without a message such as Error or An unexpected error has occurred) it may because your farm uses multiple Web front-end servers and you failed to properly configure ControlPoint for all of them.
Follow all of the instructions for Configuring ControlPoint for Additional Front-End Servers.
Parser Error Message
Could not load file or assembly "System Web Extensions. Version - ...
If you see this error as soon as you open ControlPoint, the applicable version of the .NET Framework has not been installed on all of the Web front-end servers in your farm.
Make sure the version of the .NET Framework that is appropriate for your SharePoint version is installed on all Web front-end servers in your farm. It is typically installed on the first server as part of the ControlPoint installation. You can install it on additional servers (as well as on the first server if this step was skipped during the ControlPoint installation process) by following the procedure for Configuring ControlPoint for Additional Web Front-End Servers.