Can not configure static ipv6 adress with My Cloud OS 5


since the update to My Cloud OS 5, I am unable to configure a static public IPv6 address.
After selecting the IPv6 DHCP Configuration Option for tests, it is no longer possible to change this setting.

I can click on the “static” Button and enter the address data into the popped up window, but the configuration still sticks to DHCP after leaving and reentering the network configuration.
The same happens, when I try to change the configuration to “Automatic” or “Off”.

I found the files /etc/NAS_CFG/config.xml and /usr/local/modules/default/config.xml via cli access,
but have no idea, what to change, to get the static setting.
Maybe it is possible to configure this by editing (or deleting) one of the /usr/local/modules/default/dhcp6c.conf* files!?

Any hint would be appreciated.