It looks like the interface name is missing at the link local address. Link local addresses are only valid on one interface, which is why the link local addresses have to be always specified with an interface (e.g.
fe80::1234:5678:1324:5768%eth0). So the UCR variable has to be set like this:
ucr set ipv6/gateway=fe80::1%eth0
But yes, there is currently a problem with link local addresses (fe80:: ) and the gateway configuration.
If there is no problem for you in configuring static link local addresses, you can use the following (only roughly tested) workaround:
ucr set interfaces/eth0/ipv6/dummy1/address=LINKLOCALADDR \
LINKLOCALADDR has to be replaced with an unused link local address. You can use, for example, the one currently configured at your interface. Run
ip -6 a to get a list of all configured IPv6 addresses at your network interfaces. Also, eth0 may have to be adopted to your affected network interface. dummy1 is a unique placeholder that can be chosen at will (unless it does not interfere with other UCR variable settings).
Please note that you may have problems again in setting the IPv6 gateway if the automatic DAD (Duplicate Address Detection) fails. So for this workaround you have to make sure that the configured link local address is not used twice on the configured interface.