Veeam vCenter backup - accessing incorrect IP address of VMware node

Which network is 172.16.0.x? Is this the vMotion network?
Or is it another network at the Veeam server?

@JMeixner good point, added it to original answer as well. 10.250.100.0/24 is management and 172.16.0.0/24 is vMotion. 

The cause of most NFC errors fall into 4 primary categories:

• DNS
     (The proxy or Veeam server cannot resolve the ESXi host)
• Port (902)
     (The proxy can resolve the IP, but port 902 is blocked)
• Permissions*
     (The account specified in [Backup Infrastructure] for the vCenter does not have permissions)
• File Locks
     (The file Veeam is trying to read is locked within the vSphere environment)

Did you check the permissions of the user you are connecting to the vCenter?
https://helpcenter.veeam.com/docs/backup/permissions/cumulativepermissions.html?ver=120

Hi @SZI -

You should be able to configure Preferred Networks in the Veeam Console:
https://helpcenter.veeam.com/docs/backup/vsphere/select_backup_network.html?ver=120

Hi @coolsport00 , thanks for your feedback, however this doesn’t apply to my problem as my Veeam server has only one NIC and therefore it is defined which network to use to contact vCenter. Thank you nevertheless!

I have looked through my old notes.

A similar error was caused by wrong DNS configuration.

Please make sure, that the Veeam Server can resolve all ESX Server and the vCenter to the correct IP.

Based on the errors you posted the Veeam server appears to want to route through the vMotion network which is not correct so if the Veeam servers has a proper IP on the 10.250.100.0 network then as noted check DNS.  If you can ping the hosts and vCenter from the Veeam server and they resolve on the proper network things should work and if not then you might need to fix DNS or add host file entries to the Veeam server to have it take the proper routing to the VMware infrastructure.

@JMeixner Thanks. To elaborate further on your previous post: 

• VMware’s user used for VEEAM to connect to VMware is VMWare’s administrator, that should be sufficient 
• the TCP port 902 is reachable on every ESXi host but not for vCenter (I believe this is correct)

When configuring the VMware infrastructure in Veeam, The only thing I enter to Veeam is DNS hostname for vCenter - this can be resolved by the VEEAM server (pinging to DNS name works well - vCenter is translated to correct IP address and responds to ping requests), I don’t define any of the ESXi hosts anywhere and ESXi hosts don’t have DNS name - only IP address.

Then log in to your vCenter and check how it responds to ping for the hosts.  This could also be an issue going from VC to host.  I have seen this before if you don’t use DNS servers like in a homelab (I do have DNS set up).

In my understanding all ESXi Hosts and the vCenter server have to have an IP in the ESX Management LAN.

You are correct, there is a connection to the vCenter configured only. But this means that there is a problem between vCenter and ESX Hosts.

@JMeixner The set-up is as you wrote, i.e. vCenter being in the same VLAN as are the MGMT interfaces of ESXi hosts.

If this helps, the set-up on the VMware (in terms of MGMT and vMotion networks coexistence) is as follows: 

(the screenshot is from one of three ESXi hosts, other hosts have it the same (with unique IPs per host, of course))

The response on ping from vCenter to hosts is with 0% packet drop (in tens of subsequent pings) and sub-millisecond response times. 

EDIT: 
While running a backup job on the VEEAM server (for the sake of simplicity only with one VM in it), I see this in wireshark (trace collected on the VEEAM server): 
 

I have struggles to understand why VEEAM is interested in the 172.16.0.0/24 network. The vCenter has hosts defined as follows: 

@JMeixner No need to be sorry, on the contrary - your proposal to check gateways led me to answer. The vSphere backup NFC is enabled on the vMotion and disabled on the MGMT network. I need to check with the gentleman who set this cluster up what is the idea behind. Thanks.