Was wondering if anyone was getting the same error and figured it out. Tried two servers, same results - just the stock VirtualminPro install on CentOS 7.
Requesting a certificate for DOMINNAME from Let's Encrypt ..
.. request was successful!
Configuring webserver to use new certificate and key ..
Failed to request certificate : Failed to find section protocol imap !
Under "Configurable options for Webmin Configuration" the value is:
Full path to Let's Encrypt client command /usr/local/bin
We don't use the server for standard mail. Not sure why the reference to "imap" - just trying to install an SSL on the web server.