Ok, I think I’ve got it figured out… Thanks @pixelijn for pointing me in the right direction!
Maybe this can help someone else in the future:
There wasn’t actually any PHP error happening, which is why fiddling with debug/logging settings didn’t work. 
The problem was this line in the nginx site config:
location / {
try_files $uri =404;
}
Which should be (according to Nginx issues - 500 error for media and panel 'No input file specified' - #2 by bastianallgeier):
location / {
try_files $uri $uri/ /index.php$is_args$args;
}
What I think was happening is this: I tried visiting my.devserver.ip/, but got a 404 from nginx. Not knowing what I was doing, I tried my.devserver.ip/index.php, which got through to Kirby, but there’s not content for it, so Kirby renders a 404 page, which in PlainKit just says “Error”.
I still haven’t figured out how to turn on PHP error logging, but I noticed that php --ini gives me the .ini file for the php cli, not for fpm. The correct file on Ubuntu 20.04 is in /etc/php/7.4/fpm/php.ini.