I'm trying to get SSL via letsencrypt on GCE working, but got stuck.

The cert-manager pod logs the following:

cert-manager/controller/certificates "msg"="re-queuing item due to error processing" "error"="applying acme order back-off for certificate production/khw-tls-cm because it has failed within the last 1h0m0s" "key"="production/khw-tls-cm"

What could be wrong?