@panagiotis-milios Thank you for your response, appreciate your time !
I solved the issue :
The url I was using was wrong, more precisely I was using the wrong servlet :
I was using

/dispatcherv2

while I was supposed to use

/s2sdispatcher.

I also want to point out that you are right about the "appId" below the "packetId", it is not needed.
Thank you, I hope this thread will help anyone encountering this problem !

Hmm, not really...

You can get the number of sessions created in an hour with this call: https://getbraincloud.com/apidocs/apiref/#capi-globalapp-sysgetdailycounts

But brainCloud doesn't otherwise track a concurrent session count per app.

There are certainly 3rd party analytics packages that you could link into your client that would give you that sort of information though.

Paul.