Am 01.08.2017 um 12:08 schrieb Ondrej Zajicek:

On Sun, Jul 30, 2017 at 12:22:15PM +0200, Roger Schreiter wrote:

The problem is that router 93.189.172.3 uses broadcast mode for that
network while router 93.189.172.85 uses ptp mode for it. You can use
'type broadcast|ptp' option in BIRD to force the same mode.

I guess these routers use different OSPF implementation?

Yes, this was the problem. Thanks a lot!
On the other side, there runs ospfd from OpenBSD.
When forcing the bird side to broadcast, ospf runs fine, and bird intergrated as expected into the whole system.

Regards,
Roger.