I get this message all the time especially when knocking up quick Web applications to test some WIF concept.
The solution is to add the following in the web.config:
<system.web> <pages validateRequest="false" /> <httpRuntime requestValidationMode="2.0" /> ...
It may help to add:
in the Page directive as well (This is the first line of your .aspx file).
Also, refer this discussion over on stackoverflow.