Cannot enable a script plugin in vSphere Client 5.1 on Windows 2012

We have a script plugin that loads fine in the vSphere Client 5.1 on Windows 2008/7 machines.

However when the vSphere Client is launched on a Windows 2012 machine, the legacy script plugin shows up as disabled in the plugin manager with an error:

The following error occured while downloading the script plugin from https://10.227.2.10:9443/vcplugin/service/registrationinfo: The client could not send a complete request to the server. (The underlying connection was closed: An unexpected error

occurred on a send.)

The vSphere client logs has these exceptions logged:

[ :QuickInf:P: 9] 2013-07-17 09:07:20.023  Plugins2: Processing script extension com.xyz

[ :QuickInf:P: 9] 2013-07-17 09:07:20.023  Plugins2: Downloading script xml configuration from : https://10.227.2.10:9443/vcplugin/service/registrationinfo

[ :Error   :P: 9] 2013-07-17 09:07:20.023  SSL Validation failed in WebRequestPatch.

1. System.IO.IOException: Authentication failed because the remote party has closed the transport stream.

   at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)

   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)

   at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)

   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)

   at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)

   at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)

   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost, X509CertificateCollection clientCertificates, SslProtocols enabledSslProtocols, Boolean checkCertificateRevocation)

   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost)

   at VirtualInfrastructure.Utils.WebRequestPatch.PreAuthSSL(HttpWebRequest request, Uri uri)

[ :Failed t:P: 9] 2013-07-17 09:07:20.055  System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send.

Server stack trace:

   at System.Net.HttpWebRequest.GetResponse()

Exception rethrown at [0]:

...

   at System.Net.HttpWebRequest.GetResponse()

   at VirtualInfrastructure.Utils.WebDownload.GetResponse()

1. System.IO.IOException: Authentication failed because the remote party has closed the transport stream.

...

   at System.Net.ConnectStream.WriteHeaders(Boolean async)

   --- End of inner exception stack trace ---

[ :Error   :P: 9] 2013-07-17 09:07:20.055  Plugins2: Exception while downloading from : https://10.227.2.10:9443/vcplugin/service/registrationinfo

1. System.Net.WebException: The client could not send a complete request to the server. (The underlying connection was closed: An unexpected error occurred on a send.)

   at VirtualInfrastructure.Utils.WebQuery.LocalizeWebException(WebException exception)

   at VirtualInfrastructure.Utils.WebQuery.Request(String url, Nullable`1 timeoutSecs)

   at VpxClient.Plugins.PluginManager.DownloadScriptPlugin(Extension extension, StringBuilder& sb)

The script plugin server has a self signed certificate [sha1RSA]. Importing and placing this certificate into the Trusted Root Certification Authorities store has not helped.

Any workarounds for the issue?

Thanks in advance.