Seems like there might be an outage underway on the staging server.
When responding to an ACME challenge, it doesn't look like the challenge is leaving the pending state. I've tried from fresh accounts and other ACME clients (this is affecting Let's Debug too).
Log too big to paste inline but here it is.
The authz in question: https://acme-staging-v02.api.letsencrypt.org/get/authz-v3/195157699