The value for fixed-cidr-v6 is a valid ipv6. I would have expected the problem on another level: docker desktop always runs in a utility vm (with own private network), so I would have only expected a problem to access the ULA from the host.
May I suggest posting this as issue in the Github Project for Docker Desktop for Windows: https://github.com/docker/for-win/issues. I feel the error message is a bug.