Thank you for using our services.
The problem is that - your hosting supports medium trust but your
project is not configured to run in medium trust. Another possible
reason is that - your project is configured to run under medium trust,
but your hosting provider security policy is not the default one and it
makes conflicts with Sitefinity.
You can gather more information about the trust level in ASP.NET and ASP.NET Policy Permissions and Trust Level from MSDN
All the best,
the Telerik team