Collabora Online 4.0.4. When the session is idle the editor frame does not react to postMessage API requests e.g. Close_Session, Action_Print, Action_Export. I think it's quite common for a user to stop editing the document and after a while, without even clicking inside the editor, try to download/print or just close the session. What's more, for Action_Export it already starts displaying "Downloading..." spinner, but it never ends. It should just work or automatically "wake up" the session.
Aleksander, if this issue still occurs, please open an issue over at: https://github.com/CollaboraOnline/online/issues