I upgraded from 3.5. Sometimes the stories will load, but even if they do it takes over 2 minutes. I just ran it again and it took 4 minutes to load. The page loads quickly but the grid is empty until they load. And, then about half the time I get that error message.
+ 1.455 238.184 1696 30461 POST 200 application/json; charset=utf-8 http://www.okc.biz/Sitefinity/Admin/Services/ContentService.asmx/GetContentItems