Hello.
Summarizing in the cluster the ESXi hosts version 5.5 has no problem to connect, only the ESXi hosts version 5.1 does not connect. that's right? Then you can try to upgrade one of the ESXi hosts from version 5.1 to version 5.5 this should not be difficult.
Since the vCenter server is on another network segment (10.80.1.51) you should double check that you have a connection (ping) from the vcenter server to each of the ESXi hosts that do not connect.
I will check the logs you sent, but that will take more time.
Enrique Espinel
Senior Technical Support on IBM, Lenovo, Veeam Backup and VMware vSphere.
VSP-SV, VTSP-SV, VTSP-HCI, VTSP
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