How to debug Vaadin connection problems?

Hello everyone,

we are using Vaadin 7.4.2 with server push (long polling). We never had any trouble with the API or functionality of vaadin itself, but we often get strange "hickups" with the client-server-communication, such as:

1. The client showing the message "Connection problem. Take note of any unsaved data, and click here or press ESC to continue"
2. The client showing the vaadin loading indicator bar, which extends from left to right and starts blinking, but the loading process never finishes (the server is idle during this process)
3. The server sending push messages that either never reach the client or are ignored by it (i.e. the client-side UI doesn't update)

We are really having a hard time debugging these issues. Network itself can't be the cause, because we are already experiencing these issues on our development machines where client and server run on the same local host. In each and every one of the three cases stated above, there is no error on the server (nothing in the logs, no exceptions or stack traces, not even warnings), and also no JavaScript error on the client (tested on IE10, IE11, Chrome and Firefox, all the same). The vaadin client-side debug console also doesn't show any errors. All of these issues happen randomly and are hard to reproduce.

My team and I would be very grateful for any advice on this.

Thank you,

Martin