Thanks, Gökay.
I installed WordPress in the promotions folder to maintain all promotions via wordpress, away from the main site.
To be a bit clearer, my installations of wordpress work *fine* when I first install them. It’s only after a few days that something goes wrong and the FATAL ERROR appears. I have no idea why.
I also installed manually though Filezilla, and the same issue happened a few days later (today). I’m guessing there’s something wrong with the theme. I’ve lost my database for the third time.