Recently, without any significant changes that I'm aware of, I found that my webmin instance was not accessible. It simply said "connection refused" in chrome on the default port 10000. After checking the systemctl status, it appeared that the webmin service was still running (and in fact, all of my websites were still accessible), but using "netstat -tulpn" showed that nothing was listening on port 10000. Usermin on port 20000 was working just fine.
I ended up restarting the service, which worked, but less than 30 minutes later while I was doing normal things, I got a popup in webmin that said "lost connection." Upon refresh, the same thing was happening; nothing was listening on port 10000, usermin works fine, websites work fine, etc.
It sounds to me like miniserv is crashing, however the miniserv.error logs show absolutely nothing around the time of the crash. You might also consider memory usage, but my server has 16GB and I rarely use more than 60%. dmesg shows nothing about killing any processes.
Where should I be looking to diagnose this problem?