I have no idea what might be causing it. My current theory is that a bit of the Apache server has gone mental (perhaps mod_rewrite) but it could be something in our WordPress setup too.
For now, I have reverted FT to a primitive form of URL which seems to be less problematic.
Popular is on https://freakytrigger.co.uk/?cat=2 for example.
immediate update
OH GOOD GRIEF. not even that seems to work properly (the ?cat= thing). But you can at least click on individual posts reliably. please?
update 2
I’ve reverted to the proper URLs. it really does feel like apache (it’s not rewrite, something weirder). i’ve found that refreshing eventually forces the right page to appear (sometimes 1 refresh, sometimes several tries) so something is going wrong either between supplying the URL to PHP, or in sending the correct request back. god knows.
if this does turn out to be wordpress oddness i will feel right foolish.
update 3
LOOKS LIKE WE’RE BACK IN ACTION! There may be badness cached around the internets, but fresh requests are coming out ok right now (12:40pm)