Apache - error during websocket handshake, unexpected response code: 502

I have this following setup of Apache, but its failing with following error:

WebSocket connection to 'wss://mydoain.com/socket.io/1/websocket/zpxKUhj9zN3MsTdXIkrL' failed: Error during WebSocket handshake: Unexpected response code: 502

#Server version: Apache/2.2.15 (Unix)
#Server built:   Jul 23 2014 14:17:29

NameVirtualHost *:443
<VirtualHost *:443>
  Header set Access-Control-Allow-Origin: "https://from.mydomain.com"
  SSLEngine on
  SSLProxyEngine On
  ProxyPreserveHost On

  SSLProtocol all -SSLv2
  SSLCipherSuite ALL:!ADH:!EXPORT:!SSLv2:RC4+RSA:+HIGH:+MEDIUM:+LOW
  SSLCertificateFile /etc/key.crt
  SSLCertificateKeyFile /etc/key.key
  SSLCertificateChainFile /etc/key.ca

  ServerName to.mydomain.com
  ServerAlias to.mydomain.com
  ProxyPass / http://localhost:8888/
  ProxyPassReverse / http://localhost:8888/
  #ProxyPassReverseCookiePath /MYSITE/ /
        CacheDisable *
</VirtualHost>

NOTE: Server of http://localhost:8888 is a websocket based signaling server