We apologize for the late response.
It seems there is a permission issue on the live server.
Could you try running your project and newsletter module locally? An assembly should be generated in \[YourProjectName]\Sitefinity\ServiceAssemblies
directory. Please upload it to the same location on the live server and try accessing the Newsletter module again.
Let us know about the results.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.