Are you seeing this message about your server ?
Action may be required to prevent your Let's Encrypt certificate renewals
If you already received a similar e-mail, this one contains updated
Your Let's Encrypt client used ACME TLS-SNI-01 domain validation to issue
a certificate in the past 60 days
This is an issue caused by the certbot binary being out of date. We are seeing it on a few systems, and it seems there is an easy fix for it!
Add backports by adding a line into /etc/apt/sources.list like this
deb http://ftp.debian.org/debian stretch-backports
apt-get update ; apt-get install certbot python-certbot-apache -t stretch-backports
Anything else you will need to download the binary for as per https://certbot.eff.org/
If you get stuck doing any of this, pop in a support ticket and we can help out.