Ok, answering my own question:
I can switch off the ceiling for all rooms on the floor I want to render, then it works. That's a bit annoying, but works, I guess.

Ok, that doesn't work - now when I render I see the ceiling of the floor that's visible, which is not what I want - is there an easy way to switch off the ceilings in rendering when in Virtual Visitor mode? I haven't found anything yet searching the forums.

Thanks for the detailed instructions, that's super helpful!

<t>I'd like to over time do multiple renderings and overlay them pixel-exact. For that, I'd like to store the exact camera location and viewport size.<br/>
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I can store the camera point, but that will always be relative to the currently selected item, so I have to remember which item I used ...