I am posting this because I had it happen too many times and have found it very hard to find this information on the VMWare support site. The links that I saved would either become invalid or would not point to the same thing any more.
This came from the VMWare site so if you have doubts about it, please go there and check it out.
When attempting to manage an ESX Server 2.x virtual machine, the Management Interface reports one of the following:
Unexpected response from vmware-authd: 511 Error connecting to /usr/sbin/vmware-ccagent process.
Unexpected response from vmware-authd: 511 Error connecting to /usr/sbin/vmware-serverd process.
When this ESX Server host is managed by VirtualCenter, the VirtualCenter client may also hang or fail to manage the virtual machines running on that ESX Server host. Is it possible to get past these problems?
Below are the steps you should take to restore connectivity to ESX Server virtual machines without restarting ESX Server itself to resolve the problem.
Check the free disk space on the root filesystem (the / directory).
Log into the Service Console.
Run vdf -h.
If the Use% column is at or near 100%, move any non-essential files, such as .iso files, to another location to make more space available.
Restart vmware-ccagent or vmware-serverd.
If this ESX Server host is listed in VirtualCenter, run:
ps -auwwwx |grep vmware-ccagent
If this ESX Server host is not listed in VirtualCenter, run:
ps -auwwwx |grep vmware-serverd
Find the process ID for vmware-ccagent (or vmware-serverd).
Kill the process using kill -9 nnn, where nnn is the process ID noted in the previous step.
Restart the http daemon. Run /etc/init.d/httpd.vmware restart.
One possible cause for these problems is attempting to use the Management Interface and the VirtualCenter client to simultaneously access ESX Server while the server is managed by VirtualCenter.