My best guess is that you are running the 3.5 .NET version of mojoPortal under .NET 4. Ideally you should install the .NET 4 version of mojoPortal, but you can also work around this specific error by adding this in Web.config just before the closing </system.web>
<httpRuntime requestValidationMode="2.0" maxUrlLength="560" maxQueryStringLength="2048" />
requestValidationMode="2.0" is the important part for this error
Hope that helps,
Joe