i received an error today while trying to use the vmware converter plugin for vcenter.
the error message simply said:
“a fatal error occurred. the reason for this might be a network connection failure.
the wizard will close now. please check your network connection and try again.”

the solution to this problem involved restarting the vmware converter services on the vcenter server and then closing and reopening the vsphere client.

source:  http://communities.vmware.com/message/1676872