It was Node 15. Downgrading to Node 14 worked. On a Mac, that's
rm -rf /usr/local/bin/node
rm -rf /usr/local/bin/npm
and then reinstalling Node 14.
I created two test servers, one with typescript and one with javascript.
For typescript, I had to run npm install first, then npm start, whereas with javascript npm start worked immediately.
Cheers!