I have found out what the problem was. It seems to be an issue in the ionic-platform-web-client.
In the generated code in the ionic.io.bundle.min.js file there is: e.pluginConfig.android.senderID=t.app.gcmKey
so when I run the app, always the GCM-Key is in there. Now to test I have set the senderID in this file manually and now it works.
Seems like the ID is not loaded from the package.json.
I had the same error as you. My issue was that I entered the wrong project number when I initially installed phonegap-plugin-push. To correct the number I replaced the wrong number here: plugins/fetch.json and here: plugins/android.json and I ran this command in the project directory “ionic config set gcm_key XXXXXXXXX”. I hope this helps someone in the future.