we finaly solved this problem in following way:
we are using vyos (former vyatta) for NAT 1:1 to our project networks. Veeam is trying to use vm private ip (acquired from vm tools) instead of internal ip. to solve this, we put persistent route to project isolated network, to backup servers routing table. gateway is set to some NAT address from project network.
Example: VM internal NAT ip address 172.16.1.2/private ip 10.16.1.2, backup server static route 10.16.1.0 mask 255.255.255.0 gw 172.16.1.2 if 172.16.8.50 .
This is probably only feature/bug of vyos but who cares