My hunch here is that letsencrypt issuance failed for some reason.
You might be being rejected.
Yeah, check app.yml and your connectivity to LetsEncrypt:
docker exec app bash -lc '
curl -4 -v \
--connect-timeout 10 \
--max-time 20 \
https://acme-v02.api.letsencrypt.org/directory \
-o /dev/null
'
from /var/discourse