A couple of times now, I have ran into the following error with vCenter 5.1 loaded on Server 2003 x64 when opening up vCenter Plug-in Manager:
The following error occurred while downloading the script plugin from https://vcenterserver.domain.com:8443/health-ui/scriptConfig.xml. The client could not send a complete request to the server. (The underlying connection was closed: An unexpected error occurred on a send.)

The issue in each case has been due to missing AES Cipher suites in Server 2003.  Running the following Micrsoft Hotfix has resolved it in both cases: http://support.microsoft.com/kb/948963