Connection problems since upgrade to Meteor 2.5.6

We upgraded Meteor from 2.2.1 top 2.5.6, due to an incompatibility with opensea.js. Everything works fine on the development server, but once I push the app to AWS (using mup), I am experiencing connection problems.

It should be noted that we’re not using the Meteor webapp here, but a custom DDP protocol library for Unity. The problems only occur in this scenario, not if I use the web app.

The symptom: The app manages to connect to the server and also logs in successfully. But then, all of a sudden, the WebSocket shuts down for no reason.

Were there any changes to the DDP protocol between 2.2.1 and 2.5.6?