Prueba haciendo un OVERRIDE del SessionIdManager por defecto
Código ASP:
Ver originalPublic Class CustomIDSessionManager
Inherits SessionIDManager
Public Overrides Function CreateSessionID(context As System.Web.HttpContext) As String
Return Guid.NewGuid().ToString()
End Function
Public Overrides Function Validate(id As String) As Boolean
Try
Dim testGuid As New Guid(id)
If id = testGuid.ToString() Then
Return True
End If
Catch
End Try
Return False
End Function
End Class