The VCS server is on the same subnet (the subnet is 255.255.0.0, but if we used 255.255.255.0, it would be on the same subnet too).

I can ping the adress, even by FQDN (I do have to include the domain name, pinging only the server name returns errors).

Pointing my browser to the address fails, but I can access the ip address with Windows Explorer (to copy logs,...).

I'm using both the Windows VI Client and the web interface.

Version of Java installed: 1.7.0 on one machine, 1.6.0_35 on another machine (doesn't work either).

I can't seem to find anything related in the release notes.

The agent-problem had obviously nothing to do with it.

Wait a sec. It suddenly works.

I can only imagine that my Agent-problem above WAS the problem:

I had a problem after upgrading vCenter to 5.1, described in this article:

kb.vmware.com/.../search.do

I followed the instructions, and that problem was solved. The problem with the EQL-VSM, however, not.

I did remember, though, that the release notes stated that, when esx-hosts disconnect and reconnect, the VSM has to be reinstalled.

Every time I rebooted vCenter, the hosts were disconnected, so after solving my vCenter-problem, I removed and installed VSM again.

At first, this didn't seem to help at all. I tried rebooting in single user mode to check some things out (I don't like looking in the logs by first making the dump and copying them through ftp), and I didn't see anything.

I rebooted, and suddenly the plugin works.

So in short, if you have problems with VSM, and you have (or had) the problem described in the VMWare article above, follow the instructions in the KB, and once you see your ESX hosts stay connected and the agent update stops running, try completely removing VSM (maybe make a backup first and copy it  from \\VSM-ipaddress\Database), re-deploy the .ova.

If it still doesn't work, try shutting down the application and start again.

I looked for network issues, but I can't seem to find a problem there either.

The firewall on both the vCenter as my workstation are turned off, so it can't be a firewall issue.

My first thought would be that the http-server doesn't get started, but I started in single user mode, checked the lighttpd-log, and it is started without errors.

Alas, for some reason, the error returns!

I logged on this morning, and the Plug-in fails to load again... Nothing happend this weekend so the VMWare hosts did not disconnect or anything...

A reboot just did the trick, also I went into the web address and added the Group IP and signed into the EQ. Nothing everything is working!

Sorry to resurrect this thread but Im having the same thing appear. Its fully ping'able by the network, I can even browse to the web interface. But when directing things to /vcenterplugin.xml nothing appears. (I get a 404).

A reboot of what? VCenter or the VSM Appliance?

I can currently get to the Web address of VSM Appliance and Log in but do not have the Plugin active in Vcenter / VI Client

Rebooting he VSM Appliance should do the trick.