The below error commonly occurs when you try to add or connect an ESX 4.1 host to a vCenter 4
A general system error occurred: internal error: vmodl.fault.HostCommunication
The initial connection will work and the vCenter will start to deploy its agent on the ESX, but afterwards you’ll get this error. In my case the ESX host was trying to connect to an old retired vCenter 4 which was started by mistake.
Right click on the faulty host and hit connect while connected to the vCenter 4.1 through the VMware vSphere Client.