LinkButton postbackurl Error in asp.net

Question

I want to send a Message by clicking on a LinkButton through yahoo messenger in Asp.net. Some times It works and sometimes I get this errors: Windows can not access the spesified device, path or file. You may not have appropriate permission to access this item and also Error: Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 400 This is my code in VB, What is wrong with it?

Protected Sub LinkButton2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles LinkButton2.Click
    Dim idname As String = "YMID"
    Dim message As String = "Hello, This is my YM message"
    Dim sendim As String = String.Format("ymsgr:SendIM?{0}&m={1}", idname, message)
    LinkButton2.PostBackUrl = (sendim)
End Sub