Hi Univention, friendly greetings! 
Since a few years the UCR variable ipv6/gateway supports a zone index, see:
We’re using a link-local gateway, and have multiple interfaces. Installed is UCS 5.0-10, latest errata. As fe80::1 is link-local, the kernel needs to know the outgoing interface when using it as a next hop, so we require a scope link, like so:
ucr set ipv6/gateway=fe80::1%ens19
This results in the following warning:
Setting ipv6/gateway
W: Value 'fe80::1%ens19' incompatible for 'ipv6/gateway', but setting anyway
Multifile: /etc/network/interfaces
...
The interface (ens19) is correct, the syntax is correct, and the resulting /etc/network/interfaces configuration is perfectly fine. Everything works as expected.
So my only question is:
Why does UCR still warn about an “incompatible value”?
Is this just an overly strict schema check that doesn’t yet recognize the % syntax for link-local gateways, or am I missing something deeper?
Greetings from Berlin - Lutz