Google Cloud Messaging - server connection

I wish did fix the issue. I haven't experienced that issue in my case when using GCM. Have been comparing my implementation with your current code to see if find something relevant. Will suggest try using the application context to get the instances to assure all instances are in the same context.
For InstanceID :

like below fixes the issue You need to mix both Ejabberd and FCM together, that's how all the big chat apps do it out there. For the very basics, there are 3 components: App Server connected via XMPP to FCM, Ejabberd and your client app.
When the app is in the foreground, you use Smack library to connect directly to your Ejabberd server, send messages, change user's presence, etc. The connection to your Ejabberd is kept during that time. You don't send upstream messages during this time! Once the user navigates away from your app, you close the connection. The user is now considered "Away" or "Offline". From this point and on, your App Server communicates with FCM to send downstream messages to the device using Smack library as well. On the client device: You handle the incoming message and show a notification. With Android N, users can reply directly from the notification. I assume that in this case, you would use FCM to send an upstream message to your app server since during that time, there's no active connection to your Ejabberd server. Once the user taps on the notification, the app comes back to foreground and you re-connect to Ejabberd and back to step 1.

Any of those help Try to look at the new Firebase service from Google. It is the replacement of the old GCM.
What is firebase: https://firebase.google.com/features/

wish help you to fix your issue I was getting the same Error when I tried to implement XMPP(CCS).So after going through so many site I found that to use up streaming feature of GCM (XMPP) you need to register for this service as a trail partner.
they are saying something like this

it fixes the issue I have written a code for handling such cases(basically diverting new downstream messages to a new connection)... not thoroughly tested...