Google App Engine. Channel API. The provided token is invalid. - google-app-engine

Question

I've been trying to make an application for calling browser to browser using WebRTC. I wrote a simple servlet for creating channel:
PrefClubChannelServletServlet extends HttpServlet {
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws IOException {
String channelKey = req.getParameter("userid");
resp.addHeader("Access-Control-Allow-Origin", "*");
ChannelService channelService = ChannelServiceFactory.getChannelService();
String token = channelService.createChannel(channelKey);
resp.setContentType("text/plain");
resp.getWriter().println(token);
}
}
I've deployed it in Google App Engine.
In my web application I've got a page with Java script, similar to https://apprtc.appspot.com. In my code Caller calls prepare(1), and Callee – prepare(0).
function prepare(ini) {
initiator = ini;
card = document.getElementById("card");
localVideo = document.getElementById("localVideo");
miniVideo = document.getElementById("miniVideo");
remoteVideo = document.getElementById("remoteVideo");
resetStatus();
console.log("Try to get token");
getToken();
}
function getToken() {
var token;
if (initiator) {
var xhr = new XMLHttpRequest();
var url = 'http://pref-club.appspot.com/prefclubchannelservlet?userid=GGGG';
xhr.open('POST', url, true);
// Specify that the body of the request contains form data
xhr.setRequestHeader("Content-Type",
"application/x-www-form-urlencoded");
xhr.send(url);
xhr.onreadystatechange = function () {
if (xhr.readyState == 4 && xhr.status == 200) {
token = xhr.responseText;
console.log("token = " + token);
document.getElementById("token").value = token;
openChannel(token);
doGetUserMedia();
}
};
} else {
token = document.getElementById("token").value;
console.log("token = " + token);
openChannel(token);
doGetUserMedia();
}
};
So, both Caller and Callee use the same token to open channel, and indeed they open channel, got media and made RTCPeerConnection
function openChannel(channelToken) {
console.log("Opening channel.");
var channel = new goog.appengine.Channel(channelToken);
var handler = {
'onopen': onChannelOpened,
'onmessage': onChannelMessage,
'onerror': onChannelError,
'onclose': onChannelClosed
};
socket = channel.open(handler);
}
The main problem is that event handler onChannelMessage doesn't work. I don't see any S->C: in console log. Callee doesn't see offer by Caller.
Then, I refreshed my servlet, redeployed it, and discovered I can't open channel at all. While Opening channel I'm getting Uncaught Error:
The provided token is invalid.