The following threads, when clicked, generate on my browser (Mozilla Firefox) an error message reading "The document contains no data.":

Ask Jonman Again (Daily Life)
Tell Me Your Problems! (Daily Life)
The Stylr Association (Daft)

The first two I haven't been able to access for ages; the last one is a new phenomenon, as you can tell since I very recently posted in it.

Any ideas as to the cause of this?

Not the faintest. I'm all Firefoxey (suprise suprise) and they're working perfectly for me. Which version are you running, and is it a clean install or have you upgraded through a few?

Not sure - I think the latest version, but for some reason the "About" drop-down menu is missing. I've upgraded for the last one, I believe. I poked around trying to find out, then accidently deleted all my bookmarks and gave up.

Some experimentationing reveals that this happens with all threads above a certain number of posts, this number being around or a little over 100 (explaining why I was suddenly unable to access STYLR - my own post pushed it above this limit). I should mention at this point that I had the same problem, on another computer, running IE and Norton Internet Security - but that stopped happening when I disabled Norton. I'm not actually running any internet security programs on this PC, so I don't really know what the problem could be...

Disabling Norton? Pah, you'll be lucky. Have you ever looked to see what a mess that beast makes of your registry? It's not pretty.

I really can't think of anything at the server end which would stop you being able to get at the longer threads but leave most people working fine. I suspect nasty little goblins in your computer. They can be a perennial problem.

Well, this computer's never had Norton - it does seem to be my computer, but have you any idea what exactly could be wrong?

Not off the top of my head. There's always the old chestnut of nuking any caches you can find of course. Sacrificing a chicken/goat to a dark and hungry god may help about as much though.

I'm also on firefox and I don't have any trouble viewing those pages. Did you uninstall or delete the old versions of firefox when you updated? The newest version (for windows, which I'm assuming your using) is 0.9.3.
If that's not the problem, it could be an extension interfering.