I just had an issue with vCenter 5.5 and ESXi 5.5 where I was unable to power on a VM.

The error which I was faced with was a relatively generic message based on the host connection status.

"The operation is not allowed in the current connection state of the host"

The ESXi server itself was connected to vCenter and had a number of VMs already powered on and working correctly.

However I noticed that the CPU and Memory utilisation statistics for the ESXi server were showing as zero "0" used, odd because there were 3 VMs already powered on using CPU and memory resources.

With my running VMs still powered on, I disconnected this host from vCenter (waited about 1 min), then connected the host back to vCenter.

The ESXi server CPU and memory statistics were now showing correctly for my powered on VMs.

Attempting to power on a VM is now successful.

This is the first time I have seen this host connection issue with vCenter 5.5, if it happens any more I will update my comment here and report a bug to VMware.