Hey Virtualmin devs,
You're current install of letsencrypt is broken, looks like the agreement link has been updated and the validation process is looking for the new pdf, which fails.
SO in short:
sed -i s#'https://letsencrypt.org/documents/LE-SA-v1.0.1-July-27-2015.pdf'#'https://letsencrypt.org/documents/LE-SA-v1.1.1-August-1-2016.pdf'# /usr/share/webmin/webmin/acme_tiny.py
This fixes the issue for users.
Further details can be seen here:
If you guys could roll a fix your side that would be great.