Hey,

I was searching for a reason why a a OSPFv3 interface is down and won't connect to the neighbors.
...and then I found that the Ethernet interface does not have a Ipv6 link local address.
I cannot find a reason why. All other interfaces on all my other routers do have link local addresses and OSPFv3 is working.

Can anyone help?

Is the interface part of a bridge? I know that you have to specify an administrative MAC address on a "loopback" bridge in order to active IPv6, but come to think of it, my home router's LAN interface is a bridge and I never had to specify an admin MAC.

Does the interface have an active link on it? (I think it doesn't generate any link-local addresses for interfaces that are down)

yes, port is up. no, not part of a brigde.
I took another Mikrotik router, put it on the same place and started the configuration again from scratch.
Now this new configuration is working as expected.