Using webmin 1.850, after upgrading to Debian 9, I could not renew letsencrypt certs. I tracked it down to this bug:
https://github.com/diafygi/acme-tiny/issues/167
(Error shown on screen included the text "detail": "Error creating new cert :: Authorizations for these names not found or expired: ...."
Webmin include a version of acme-tiny which does not cope with openssl 1.1.0.
Once I updated /usr/share/webmin/webmin/acme_tiny.py per that ticket, everything worked.
Thanks,
Alex
Thanks, this is a known issue - we will be including this fix in the next Webmin release.