Iron router and undefined routes crash web socket connection

I am working on an app for a long time now (Meteor version 1.5.1 now) which is still using iron:router, and since today undefined routes crash with the following message in the console

WebSocket connection to 'ws://localhost:3000/sockjs/832/6ohpvjgz/websocket' failed: WebSocket is closed due to suspension

I have configured a default template for notFound, but the browser reloads because the WebSocket gets terminated.

Is anybody else seeing this?