cPHulk Can't Clear Blocks due to ip6tables

Symptoms

Clearing cPHulk blocks encounters errors similar to the following if IPv6 is broken, disabled, or not supported by the kernel.

[iptables] /sbin/ip6tables --list-rules cphulk failed: ip6tables v1.4.7: 
can't initialize ip6tables table `filter': Address family not supported by protocol 
Perhaps ip6tables or your kernel needs to be upgraded. : 
The subprocess reported error number 3 when it ended. 
at /usr/local/cpanel/Cpanel/IpTables.pm line 416.

Description 

These errors are typically caused by a broken IPv6 configuration, IPv6 being disabled, or IPv6 not being supported by the kernel.

Workaround

1. Access the server's command line as the 'root' user via SSH or "Terminal" in WHM.
2. Verify that the server's IPv6 configuration is correct and that the server can reach the default IPv6 gateway.
   Please note that you may have to contact your network provider to confirm that the IPv6 configuration is correct.
3. Update the kernel.
   • CentOS 7, CloudLinux 7

     yum update kernel

   • RHEL 8/9-based OS

     dnf update kernel

   • Unbuntu

     apt update

     apt upgrade kernel

4. Reboot the server to complete the kernel update.

   shutdown -r now

Additional resources

How to diagnose IPv6 issues.