The following exception occurs in ASP.NET (version: 2.0.50727.1433) when you trigger an event on an ASP.Net object that has no ID set, eg clicking on a LinkButton: System.ArgumentException: Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true... in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server...
This will actually remove the your workspaces - unlike the VS/TFS Source Control Explorer which simply does not display them - duh! From the VS command prompt: tf workspaces /remove:*...