Thanks, spider, though I should have mentioned it does it even in fresh scenes, and just a groundplane. I think I've found a possible reason, though: when Vue crashed once, it went to "Compatibility Mode" - according to a post on e-onsoftware, when it does this certain features like Automatic Preview are disabled. And in the Material Editor if you see a small exclamation mark in the window it means this too has been disabled. To fix this, go into Otions>Preferences and choose "Restore defaults". Preview options for scene and material should work again.

I've done that and some previews seem to work, but again the bloody scene preview needs to be clicked manually sigh. I guess I'm too used to Vue4.