Assuming you're using ESXi 4.0+, as long as you can ping ESX from controller VM, the only case that you may get the this error is a wrong credential was given and the session was interrupted. Please try to restart tomcat server by running the following command on the controller VM: `
/etc/init.d/tomcat5.5 restart`. And then try to add ESX host again. If the problem persists, please send log files to our maillist and provide your ESXi version.
To download the log files, you may simply scp from the controller VM. On Windows, you'll have to install some scp software such as WinSCP. On Linux, you may simply open a terminal and use scp command.
Thanks,
Chien-Chia