Proxy error on get/post- request

[Stefan Elfvinge]

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /makeCase.
Reason: Document contains no data

If there is a general explanation to the above, could anyone tell me it means. I get when one servlets doGet() or doPost() is supposed to work properly. But all I get is the proxy-error. I have other servlets written and configured the same way and they work fine???
/thanks