Ok. This might help ->
Find the line in callback.aspx.vb -->>
strResult += "FF_" & fldName & ":*:" & Request.Form(fldName)
and replace it with this -->>
strResult += "FF_" & fldName & ":*:" & HttpUtility.UrlEncode(Request.Form(fldName))
Basically we just want the form fields values to be URL encoded first before we pass them to PayPal.