Background
To troubleshoot Webmin or Virtualmin starting, one has to look at a file called miniserv.error. Virtualmin is made by the same team as Webmin, so the two products are tightly coupled. So if you’re running into a situation where Webmin or Virtualmin isn’t starting, you typically have to look at the contents in that log file.
There are actually three files, but only miniserv.error will give you clues as to why the service is not starting.
The three log file locations are:
/var/webmin/miniserv.error
/var/webmin/miniserv.log
/var/webmin/webmin.log
Miniserv.error has no information or is empty
If miniserv is empty but you are still unable to start Webmin, then do this:
# /etc/webmin/start
This will produce the newest error that is causing the service to stop from starting. Here is an example output of an error that stops it from starting:
Starting Webmin server in /usr/libexec/webmin Failed to open SSL key /home/site.com/ssl.key at /usr/libexec/webmin/miniserv.pl line 4467.
Of course the error could be anything but focus on repeatedly doing /etc/webmin/start to get the ball rolling.
The error above about ssl.key and ssl.cert files that were missing was on a perfectly stable server. The server had an uptime of 202 days and there were no changes made in this time. The server was however set for automatic updates – at this point our belief is during an automatic update Virtualmin updated itself and somehow thought the configuration of the ssl certificates had changed. Our solution was to recreate just the key and cert file across to get Webmin / Virtualmin to start again.
Reference
https://virtualmin.com/node/45346