I had done a trial of the vKernel vOperations Suite, and it installs itself as a plugin for vCenter. When the trial was finished and I uninstalled it, it was still being listed as available plugins in vCenter, but of course it couldn’t find it. Below are instructions for how to remove a vCenter Plugin
Note to self: Don’t install IIS or anything that requires IIS on the vCenter server. BAD BAD BAD.