"Sign in as a Different User" component does not work on a SharePoint website that is published by Forefront TMG 2010

Update information

The software update that is required to resolve this issue is described in the following Microsoft Knowledge Base (KB) article:

2498770 Software Update 1 rollup 3 for Forefront Threat Management Gateway (TMG) 2010 Service Pack 1
To resolve this issue, apply this update, and then run the following script on any array member.

Note This script enables SharePoint Server (SPS) publishing that uses FBA and authentication delegation that uses KCD. If you want to enable SPS publishing that uses integrated authentication, and if you want authentication delegation to use KCD, run this script. Then, run the additional script that immediately follows: Const SE_VPS_GUID = "{143F5698-103B-12D4-FF34-1F34767DEabc}"
Const SE_VPS_NAME = "EnableSharepointSignIn"
Const SE_VPS_VALUE = true

Sub SetValue()

' Create the root object.
Dim root ' The FPCLib.FPC root object
Set root = CreateObject("FPC.Root")

'Declare the other objects that are needed.
Dim array ' An FPCArray object
Dim VendorSets ' An FPCVendorParametersSets collection
Dim VendorSet ' An FPCVendorParametersSet object

' Get references to the array object
' and to the network rules collection.
Set array = root.GetContainingArray
Set VendorSets = array.VendorParametersSets

On Error Resume Next
Set VendorSet = VendorSets.Item( SE_VPS_GUID )

If Err.Number <> 0 Then
Err.Clear

' Add the item.
Set VendorSet = VendorSets.Add( SE_VPS_GUID )
CheckError
WScript.Echo "New VendorSet added... " & VendorSet.Name

Else
WScript.Echo "Existing VendorSet found... value- " & VendorSet.Value(SE_VPS_NAME)
End If

if VendorSet.Value(SE_VPS_NAME) <> SE_VPS_VALUE Then

Err.Clear
VendorSet.Value(SE_VPS_NAME) = SE_VPS_VALUE

If Err.Number <> 0 Then
CheckError
Else
VendorSets.Save false, true
CheckError

If Err.Number = 0 Then
WScript.Echo "Done with " & SE_VPS_NAME & ", saved!"
End If
End If
Else
WScript.Echo "Done with " & SE_VPS_NAME & ", no change!"
End If

End Sub

Sub CheckError()

If Err.Number <> 0 Then
WScript.Echo "An error occurred: 0x" & Hex(Err.Number) & " " & Err.Description
Err.Clear
End If

End Sub

SetValue If you want to enable SPS publishing that uses integrated authentication, and if you want authentication delegation to use KCD, also run the following script: Const SE_VPS_GUID = "{143F5698-103B-12D4-FF34-1F34767DEabc}"
Const SE_VPS_NAME = "UseOnlyNTLMForWindowsAuth"
Const SE_VPS_VALUE = 1

Sub SetValue()

' Create the root object.
Dim root ' The FPCLib.FPC root object
Set root = CreateObject("FPC.Root")

'Declare the other objects that are needed.
Dim array ' An FPCArray object
Dim VendorSets ' An FPCVendorParametersSets collection
Dim VendorSet ' An FPCVendorParametersSet object

' Get references to the array object
' and to the network rules collection.
Set array = root.GetContainingArray
Set VendorSets = array.VendorParametersSets

On Error Resume Next
Set VendorSet = VendorSets.Item( SE_VPS_GUID )

If Err.Number <> 0 Then
Err.Clear

' Add the item.
Set VendorSet = VendorSets.Add( SE_VPS_GUID )
CheckError
WScript.Echo "New VendorSet added... " & VendorSet.Name

Else
WScript.Echo "Existing VendorSet found... value- " & VendorSet.Value(SE_VPS_NAME)
End If

if VendorSet.Value(SE_VPS_NAME) <> SE_VPS_VALUE Then

Err.Clear
VendorSet.Value(SE_VPS_NAME) = SE_VPS_VALUE

If Err.Number <> 0 Then
CheckError
Else
VendorSets.Save false, true
CheckError

If Err.Number = 0 Then
WScript.Echo "Done with " & SE_VPS_NAME & ", saved!"
End If
End If
Else
WScript.Echo "Done with " & SE_VPS_NAME & ", no change!"
End If

End Sub

Sub CheckError()

If Err.Number <> 0 Then
WScript.Echo "An error occurred: 0x" & Hex(Err.Number) & " " & Err.Description
Err.Clear
End If

End Sub

SetValue