Thanks Seeker. I've just done about 3hrs of tests with various combinations of saving, loading, new file, etc whilst the Viewport is open, and cannot get the problem to occur. If you can find a way to reproduce the issue, that would help greatly.I was trying to define the common steps the few times this occurred but as I said the only common denominator seems to be opening a new Revit file (project or family) while the plugin is open and rendering, resulting in the render window closing. As also mentioned though, other times I re-opened the plugin after the same activity it was still fine... I get the idea that it can handle a certain amount of these events but then gets 'confused' (what do you say, am I a software development candidate of what!?). Sorry that I can't offer any more insight than that. Hopefully the 2 Revit model files will help - sent them by PM about an hour ago.
Paul