Follow directions in /etc/rsyslog.d/20-ufw.conf and add '-' before "/var/log/ufw.log".
After modification, the file should contain the following:
# Log kernel generated UFW log messages to file
:msg,contains,”[UFW ” -/var/log/ufw.log
# Uncomment the following to stop logging anything that matches the last rule.
# Doing this will stop logging kernel generated UFW log messages to the file
# normally containing kern.* messages (eg, /var/log/kern.log)
& ~
