The quick way to debug this is to use Chrome and click More Tools -> Developer Tools. Click console and you might be able to find out the “500” exception as below.
Another quick way to what exactly is the issue, you could call the web service like http://spsbx08/sites/DocCenter/_vti_bin/client.svc in this case to see the real error.
“Memory gates checking failed because the free memory (214859776 bytes) is less than 5% of total memory. As a result, the service will not be available for incoming requests. To resolve this, either reduce the load on the machine or adjust the value of minFreeMemoryPercentageToActivateService on the serviceHostingEnvironment config element.”
In this case, the issue is the search services are taking too much memory on the server. We could restrict the service not to take too much memory or bounce the server as short term solution.