This line is part of the WG config file, which breaks the configuration file.
Removing the IPv6 (,fd0) makes the config file and VPN work fine. Changing the IPv6 to fd0:: makes the configuration importable, but the VPN does not work.
I can confirm this issue on my Flint 2 with 4.7.0 (non-op24). It is necessary to manually edit the address within the config file. After this step the configuration works flawlessly.