vCenter uses an Internet Explorer control to display plugin tabs. Thus, any IE settings have a direct impact to such plugin pages. Most of the time the behavior you mentioned is caused by incorrect proxy settings in IE.
Please open IE and go to Internet Options->Connection->LAN->Proxy and check what is configured there. If you use a proxy, you may have to enter an exception for the host where eCM runs on. You may have to add 'localhost' and '127.0.0.1' as an exception as well.