ASP.NET versions 1.1 (and higher) contain the feature request validation, which prevents the server from accepting content containing un-encoded HTML. This feature is designed to help prevent some script-injection attacks whereby client script code or HTML can be unknowingly submitted to a server, stored, and then presented to other users. We still strongly recommend that you validate all input data and HTML encode it when appropriate.
The error you received is because your forms are not validating input and thus ASP.net is throwing this error.
Details on how to workaround this error are available at: