I also don’t know what error logs to look for to diagnose the problem. Any ideas?
You can see the Apache error logs in either /var/log/apache2/error.log, or /var/log/httpd/error_log, depending on your distro.
The Webmin/Virtualmin error log is in /var/webmin/miniserv.error.
Definitely look there first, but if you started seeing an issue with multiple services at the same time, you may want to do a sanity check with things like disk space “df -h”, memory “free -m”, and kernel messages “dmesg | tail -30”.
So, it turns out that one of my virtual hosts ended up with a cert file that was blank. I had to comment out the correct line in the apache conf file, re-issue the cert from LetEncrypt, then uncomment the line and that seems to have fixed everything.