We are sorry to hear that you are experiencing problems with out product.
In order for us to be able to investigate your problem, we will need some more information. Can you open a support ticket and attach your web.config file in it? We will also need to know which is the exact version of Sitefinity you are using and the steps to replicate the issue so that we can be sure to end up with a proper response.
Thanks for your cooperation and excuse us for not being able to resolve the issue right away!
the Telerik team