And now it's not working again.
It seems as though it has something to do with the length of the initial post to the topic. If it's long I can't even Preview. If it's just a couple of lines, Preview works just fine (I'm not bothering to test with Submit as then we'd wind up with way too many test discussions).
I have to disappear for a while, but when I get back, I'll try some more tests.
If someone else has the chance, can they copy and paste in a page or two of text as their initial post to a new topic and see if they can Preview it?