Thank you for the quick answer. As far as I can tell, the firewall configuration looks good:
ufw status
Status: active
To Action From
22 ALLOW Anywhere
80 ALLOW Anywhere
443 ALLOW Anywhere
22 (v6) ALLOW Anywhere (v6)
80 (v6) ALLOW Anywhere (v6)
443 (v6) ALLOW Anywhere (v6)
Also I'm not sure why this issue suddenly popped up. It worked fine for the last couple of years...