What causes unreachable OSPF neighbor?

[ico]

Hello,

I have 10 routers talking OSPF on a switch on network 10.10.4.0/24. Now 
I want to add 11th and am unable to make it work. I see the new router 
in neighbors, also other neighbors on the new router. The new router 
doesn't get any routes from others, because others see it as unreachable 
(10.10.4.45 is the new router, 10.10.4.48 is one of old working routers):

   bird> show ospf topology all
   ...
   router 10.10.4.45
     unreachable
     network 10.10.4.0/24 metric 10

   router 10.10.4.48
     distance 10
     network 10.8.253.0/26 metric 50
     network 10.10.4.0/24 metric 10
   ...

On the new router - all old routers are unreachable:

   bird> show ospf topology all
   ...
   router 10.10.4.45
     distance 0
     network 10.10.4.0/24 metric 10

   router 10.10.4.48
     unreachable
     network 10.8.253.0/26 metric 50
     network 10.10.4.0/24 metric 10
   ...

The normal traffic (ping, ssh) works of course, IP configuration is 
currently at minimum (single interface, IP 10.10.4.45/24). OS is Linux 
5.10, bird 2.0.7.

Thanks for any help with this.

ico