When running EPiServer UI in .Net 4.0 the request validation mode has to be set to 2.0. This is described in the article
EPiServer CMS 6.0 now supporting ASP.NET 4.0Example
In web.config
<configuration>
...
<location path="ui">
<system.web>
<httpRuntime requestValidationMode="2.0" />
For MVC
Mark the post back action method with ValidateInput(false)
[HttpPost, ValidateInput(false)]
public ActionResult Index(WikiPage currentPage, WikiViewModel m)
{
...