I had that problem on one of my machines a while ago. I believe I had to disable IPv6 in ufw for it to start working again. Let me see if I can find out how to do that again, I can't remember.

Edit: try this:

To disable ipv6, edit /etc/default/ufw and change IPV6 to “no” (without quotes)

# Set to yes to apply rules to support IPv6 (no means only IPv6 on loopback
# accepted). You will need to ‘disable’ and then ‘enable’ the firewall for
# the changes to take affect.
IPV6=no