If you’re adding an ESX 4.1 host to a vCenter Server 4.0 installation (or anything lower than vCenter 4.1 for that matter), you’ll run into the following error message, which prevents managing the ESX 4.1 host through vCenter. The initial connection will work, it will start to deploy the agent, but afterwards you’ll get this:
A general system error occured: internal server error: vmodl.fault.HostCommunication
You might be prompted to blame DNS resolution or Date/Time settings, but if you’re running the previous version of vCenter Server, you won’t be able to add the new ESX 4.1 host to your existing vCenter 4.0.
Solution, so far, is to either upgrade your vCenter Server installation, or downgrade / reinstall your ESX host. If you’re going to upgrade your vCenter, read the vSphere Upgrade Guide. Also note that vCenter 4.1 is x64 only, there’s no longer a 32-bit version.
Once you’ve upgraded vCenter, remove the host from the inventory, and re-add it. Choosing “connect” from the right-click dropdown menu on the host, likely won’t solve it. The host needs to be removed first.