Fixed the problem: in General Settings, I had my WordPress Address set as domain1.com/folder/ instead of domain2.com because apparently I’m an idiot. The more you know!
After a bit more fiddling around I’ve gotten things to work again when I changed the Permalink setting to default, but changing it back again to Day and Name produces the 500 errors across the board again. I prefer Day and Name so whatever helps that work again…