This appears to have been an issue with CloudFlare. I forgot my DNS was running through it. It works when I deactivate cloudflare. Thank you for your work and your response.
I have, yes. I tried disabling all plugins.
Webfinger responds correctly – if I put that author link in my browser it works, and if I curl it with POST it works… but a GET gives that 500 error on just that one account, it seems.
