can you confirm that you are running Sitefinity 3.6 built against .NET 3.5 framework?
The problem seems to be in some Ajax functionality, and Sitefinity 3.6 (built against .NET 2.0) uses system.web.extensions 1.x, while Sitefinity 3.6 (built against .NET 3.5) uses system.web.extensions 3.5.
Judging by your system setup, it would look like you are running .NET 3.5 on that machine.
We are waiting for your response.
the Telerik team