Error Message
To access the Advanced GRE Web Plug-in from VMware vSphere web client ,
Cell Manager is selected, the User name and Password are specified correctly ,
but the Mount Proxy Selection fails by the error "Mount proxy host is not reachable"
although the Mount Proxy drop down list should show the available mount proxy server
for the selected Cell Manager .

Cause
The error occurs because vCenter server can not resolve the hostname of Mount Proxy from IP address . (Reverse lookup)
Even though the hostname of Mount Proxy can be resolved from IP address , the hostname should be exactly the same with the one in cell_info file .

- vsphere_client_virgo.log
[run] Starting vmwaregre agent process
[remoteStartAgentEx] vcenter hostname: <hostname>
[remoteStartAgentEx] targetHostname: <IP Address of Mount Proxy>
[remoteStartAgentEx] Caught 'InvalidParameters' exception, msg: Entry of mount proxy: <IP Address of Mount Proxy> doesn't exist!
Error: AgentStartFailed
[sendAgentRequest] Exception in sendAgentRequest. java.lang.NullPointerException: null

Workaround / Fix
The error can not be avoided by using hosts file .
In the case that DNS is not available , the error can be avoided by adding an entry of Mount Proxy's IP address in cell_info file as a workaround .