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Verify that the 800xA Service Account has been applied correctly in the misbehaving server. In any case, I recommend rerunning the System Software User Settings from the Configuration Wizard.
DNS is not used by 800xA in 5.1 and forward; RNRP updates the "hosts" file with all nodes. It does not hurt to quickly browse the hosts files with eg notepad and rule out a bad IP has snuck in.
The node administration structure has an object for each server in the system. Check the bad server's Node Definition Aspect and verify correct name is listed. Try pressing the "Connect Node" button. The operation must succeed, or else something is wrong with IP, name, authentication (service account), network, etc.
Last, verify that Windows Firewall has been configured for 800xA. You may try by turning it off. The firewall is easily setup from the firewall wizard on DVD1.