I guess I will come out of "retirement" and grab me some points although I never really get any points, since I usually post solutions without responding to questions. Anyway, I have your answer because this is something that tripped me up before and then this past weekend it happened again and I decided to just search and help any other lost souls.
Solution? Quite stupid actually.
Basically it looks like the client gets tripped up during some update because I had Update Manager running and then I had updated my ESXi boxes before, so everything seems to work fine but sometimes you will get this weird error. You can access the web page from the host server itself but you cannot log in because you get "Web Service not available" but then you can still see the clusters and hosts defined within vCenter but you cannot connect with Client or Web. The following steps are for the vCenter Host Server:
Basically leave everything alone.
UNINSTALL the vSphere client.
Reboot.
Then go to web interface on host machine http://localhost or http://127.0.0.1
Click on DOWNLOAD vSphere Client
Install the client.
Then try connecting.
WHAM!!! It connects. Not sure why but I am guessing that vCenter gets updated or something and then the client is not able to re-connect and I had the client installed from before and on another machine. Not sure if maybe I had the wrong version but if you install from the version provided by vCenter then it works. All I care is that I am able to connect again...
Good Luck...
Now give me points...
:smileylaugh: