Firewall cannot be activated through Parallels Plesk Panel interface:
/opt/psa/var/modules/firewall/firewall-new.sh failed: ip6tables v1.4.4: invalid mask '' specified
The problem is caused by error in product: if rule contains non-standard netmask (i.e. not /8, /16 or /24), Firewall Module mistakenly recognizes address as IPv6 and generates incorrect rule:
/sbin/ip6tables -A INPUT -p tcp --dport 22 -s / -j ACCEPT
The problem is under investigation by Product Development and will be fixed in future releases.