When you upgrade a VMware vSphere environment the following error can occur when you just updated the VMware vCenter server.
In my case this was an upgrade of VMware vCenter 4.1 to VMware vCenter 5.1 with VMware vSphere ESX 4.1 hosts. In the installation of vCenter I checked the option to upgrade the vCenter agents on the hosts automatically. After the installation of vCenter I checked the status of the hosts. The following error was showing.
Reading both KB articles my first try was to manually restart the vpxa agent and the management agents. See VMware KB1003490
When reading the different articles my ESX host was already unreachable for my vCenter server. Communication to the host was no problem.
So I started SSH and restarted the management Agents. Then I restarted the vpxa agent service. Both where successful. Now select Connect in vCenter. The installation of the new vpxa agent started again and now it was successful after a couple of minutes. Problem solved.
If there are still problems, see VMware KB1003714 for a manual installation of the vpxa agent.