active session create problem in cortex

Hi All,
I am trying to create a session in cortex.
I am using c# with websockets and newtonsoft json.net
I can query headsets, query logged users, get full authorization, and create a session in “opened” state. No problem.
But When I try to create a session in “active” mode, I get the following RPC error:

-32603
Internal error

Here are my parameters:

createSessionReq.parameters = new ParamsCreateSession {
_auth = _AuthorizationToken,
status = fActive ? “active” : “open”,
headset = HeadSetID,
project = “Test Cortex”,
title = “test1”,
subject = “cuong”
};

Any ideas as to what can be the reason?
Regards,