Spanish multipart error 500

Dim j As HttpJob
    
j.Initialize("", Me)
j.Download("http://xxx.yyy.com/AWWebAPI/api/aw/awa/GetAWAReservation?Hotel=H06&Localizador=42652447")
Wait For (j) JobDone(j As HttpJob)
    
    If j.Success Then
        
        Dim jp As JSONParser : jp.Initialize(j.GetString)
    
        Map = jp.NextObject
        
        Log(Map)
        
        Dim jGen As JSONGenerator : jGen.Initialize(Map)

        Dim root As Map
        
        parser.Initialize(j.GetString)
        root = parser.NextObject
        
        root.Put("Hotel", "H06")
        
        Dim j1 As HttpJob
    
        j1.Initialize("", Me)
        j1.PostMultipart("http://xxx.yyy.com/AWWebAPI/api/aw/awa/PostAWAReservationGuests",root,Null)
        Wait For (j1) JobDone(j1 As HttpJob)

    Dim Mensaje As JSONGenerator

    Dim credentials As Map   
    credentials.initialize

'    credentials.Put("domainId","")
    credentials.Put("login","NpPlusOneUser@gmail.com")
    credentials.Put("passwd","562389784512")
    
    Mensaje.Initialize(credentials)
    
    Dim Job As HttpJob
    Dim urlx As String="http://www.ultra.net/apirest/ws/"
        
    Job.Initialize("job", Me)
    Job.PostString(urlx,Mensaje.ToString)
    
    Job.GetRequest.Timeout = DateTime.TicksPerSecond * 45
    Job.GetRequest.SetHeader("User-Agent", "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:19.0) Gecko/20100101 Firefox/70.0")
    Job.GetRequest.SetContentType("application/json;charset=UTF-8")
    
    Log( "Enviando a server..." )