Hi Harsha,
This error would happen only when there is no connection happening to your Mobile Server via your reverse proxy.
BI platform does not needs to be exposed, its just your Mobile Server which you should be able reach with the reverse proxy.
Easy way is to see of there is any traffic coming to your Mobile Server system using tracing tools like fiddler or Wireshark. If not then redirection with the reverse proxy is not proper or there is some problem with the connection URL.
Regards
Vikas