Thank you mrz for your answer. The situation is like this:
The router I found out to be causing the problems is our VPN gateway. Lets say that we have a local address for VPN endpoint and it is 192.168.1.1. VPN clients get IP's 192.168.1.2, 3, 4, etc. VPN gateway has its own public address, but is also connected through eth2 adapter to our core router (which has its own public IP, but from the same /29 range as VPN endpoint. Public routes are not pushed through OSPF).
So the OSPF network rule is: 192.168.1.0/24 on vpn endpoint site and also on client sides. As soon as I add these routes to VPN router I start getting route ospf error spams. If I disconnect the VPN router from our network I don't see these errors on any other router but VPN endpoint. If I disconnect all but 1 VPN clients on VPN endpoint router = no errors. Anything else = errors. It seems that this could be something like same IP on multiple interfaces, but even if I change each VPN connection range to it's own subnet (192.168.1.0/30, 192.168.1.4/30,...) - so no VPN interface is having the same IP, I still get this error.
Closer, but still lost