Anitha, Are other users able to see the Discussions? Is this work item in an open or closed project? How is the user getting to the work item, do they search for it, do they navigate to the project then go to it through the menu items (Plan > User Story, Portfolio > Portfolio Items > Features, etc.)? You are a Sub Admin, do you get the same error? If you reproduce their workflow, do you get the same error?
-Edwin