So it seems there are various ways and means of receiving a ‘Server communication lost, closing document message’.

If you have two different machines with the same user cal viewing the same document this kills the session to use for the other machine.

If you have two different browsers(ex. FF and IE) with the same user cal viewing the same document this kills the session to use for the other client.

Or if you have the same browser(EX. IE) with Ajax and other with plug-in, and with the same user-cal, this also kill the session to use for the other client.
I find this annoying, especially as we start to roll out more iphone\ipad qlikviews. This means if a user pulls it up on their ipad they can kill their desktop session
(with a lame error that makes it look like the server crashed)
Another way that you can cause this is with a qlikview server timeout setting. To change the values open up the QEMC, system Tab, setup, click your QVS,
then performance tab and adjust your Possible session timeout: value and maximum inactive session time: