I mean Administration >> Settings >> Advanced >> Toolboxes >>... where you register the control and its location on the server.
Administration >> Settings is represented by configuration provider ( xml file storage) which stores the data inside App_Data/Sitefinity/Configuration
The error you see means that the path you used to register the control is not correct. You see the widget in the list, because you have "added" a new toolbox item - new record in the configuration, but the config element does not know whether the path the control is correct or not
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