NBU has to use "hosts" file?

OK, so now check the system files and make sure it is set to use DNS before local files.

Personally - I have had this same problem before, and since it DOES connect using /etc/hosts, it should not be a networking issue. I just added that one host to /etc/hosts and did not waste any more time on it.

Just a thought - 

Mostly this happens with multi-NIC systems, where traffic comes in one NIC and for some reason, returns via another path. Master definitely sees a difference sending to client-backup and getting a response via client instead. I have had a problem before on a linux box, I had to do this:

Here is the official link describing how to change RHEL6 and RHEL7 to support loose asymmetrical routing:

https://access.redhat.com/solutions/53031