This problem might occur with your vCenter Server Appliance. But it could also happen with other Linux-servers. It might happen when you move the virtual machine, clone it, restore it, recover it from replication or perform any other action where the MAC Address of the virtual machine changes. This is what you would see on the console of the virtual appliance:
After deploying the vCenter Linux Appliance with a default and embedded Single Sign-On configuration you can login to vCenter with two user accounts: root and Administrator@vsphere.local. If you have Active Directory in your environment you might want to add that to your vCenter-server as an identity source for Single Sign-On.
When you need to import a vCenter Linux-based appliance there are two clients that you can use to perform this task. Your best choice is the vSphere Web Client but that will only be available if you already have vCenter running. So that will only be possible for a second or more vCenter server. When you need to import your first vCenter appliance the client you must use is the vSphere Client on Windows. (How to do that is explained here.)
In this article I will explain how to import the vCenter Linux Appliance and customize it with a fixed name and IP-address.
It is common to run vCenter as a virtual machine. When the VM runs inside a vSphere cluster with DRS enabled configured to automatically migrate virtual machines you might loose track of where the vCenter-VM is running. When you need to troubleshoot the vCenter VM you might need to use the vSphere Client to manage that VM, but on which cluster node does the VM run? Therefor it might be a good idea to always run the vCenter VM on the same ESXi host.