After having upgraded to 3.71 I started to get hourly error mails stating:
Day '' out of range 1..31 at /usr/share/webmin/virtual-server/feature-mail.pl line 3129
The subject was: "Cron <root@uhura> /etc/webmin/virtual-server/bw.pl"
I use Postfix and re-checked the configuration without issues. Although I did restore multiple servers from backups yesterday (complete recreation), I only got the error mails after the upgrade.