When going to the Access and Passwords screen and setting the Server Access to "Local and Intranet Access" it creates a .htaccess file that is correct for ipv4, but only allows localhost for ipv6.
The fix for this seems to be adding the following entry to the ipv6 Allow rule if the user wants to allow Intranet Access: FE80::/64
Eg: Allow from ::1 FE80::/64
I'm not sure if there are multiple localhost ranges for ipv6, but that is the main one.
This is needed in scenarios where you want to access the web server using the server name and the os or dns entry defaults to ipv6.