I created a brand new VPS instance with Ubuntu Server v18.04.3, on a linode, and ran the Virtualmin GPL install script with no issues to get a fresh install for my primary VPS server.
I then proceeded to restore 2 websites, one with about ~500 megabytes of files in the backup, and the other with about ~75 megabtyes of files within the backup. At this point, the webmin mini-server crashed because disk quotas would not allow the ssl.key files to fully write into the webspace (other than as zero bytes), AND webmin had already added these keys to it's /etc/webmin/miniserv.conf configuration file (when restarting via CLI, webmin crashed with an invalid key error pointing to the ssl.key in the webspace I was restoring).
I adjusted the user's disk quotas, and this webmin crashing issue went away. BUT I realized the MySQL server would not start without crashing as well, UNTIL I COMPLETELY DISABLED ALL DISK QUOTAS, at which point the MySQL server started just fine. I re-ran the website restorations, and everything ran smoothly with zero issues.
Just reporting this as an apparent bug on fresh installs with large website restore files. I'm no longer experiencing any issues after disabling disk quotas, and I have no need to re-enable them. Thanks for the great ope source server software, been using it for many years with minimal issues.