Script for File upload in google drive - Error encountered: An unexpected error occurred - file-upload

Question

I use a script to upload document into google spreadsheet and automatically put link to it into current cell. Since yesterday all was gone fine, but since this morning i receive this generic error:
https://docs.google.com
Error encountered: An unexpected error occurred
This is the code:
// upload document into google spreadsheet
// and put link to it into current cell
function onOpen(e) {
var ss = SpreadsheetApp.getActiveSpreadsheet()
var menuEntries = [];
menuEntries.push({name: "Accedi al modulo per allegare un file...", functionName: "doGet"});
ss.addMenu("Carica scheda di rilevazione dati...", menuEntries);
}
function doGet(e) {
var app = UiApp.createApplication().setTitle("FORM per il caricamento della scheda di rilevazione dati");
SpreadsheetApp.getActiveSpreadsheet().show(app);
var form = app.createFormPanel().setId('frm').setEncoding('multipart/form-data');
var formContent = app.createVerticalPanel();
form.add(formContent);
formContent.add(app.createFileUpload().setName('thefile'));
// these parameters need to be passed by form
// in doPost() these cannot be found out anymore
formContent.add(app.createHidden("activeCell", SpreadsheetApp.getActiveRange().getA1Notation()));
formContent.add(app.createHidden("activeSheet", SpreadsheetApp.getActiveSheet().getName()));
formContent.add(app.createHidden("activeSpreadsheet", SpreadsheetApp.getActiveSpreadsheet().getId()));
formContent.add(app.createSubmitButton('Invia ed archivia scheda'));
app.add(form);
SpreadsheetApp.getActiveSpreadsheet().show(app);
return app;
}
function doPost(e) {
var app = UiApp.getActiveApplication();
app.createLabel('sto salvando...');
var fileBlob = e.parameter.thefile;
var doc = DocsList.getFolderById('XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX').createFile(fileBlob);
var label = app.createLabel('file caricato con successo');
// write value into current cell
var value = 'hyperlink("' + doc.getUrl() + '";"' + doc.getName() + '")'
var activeSpreadsheet = e.parameter.activeSpreadsheet;
var activeSheet = e.parameter.activeSheet;
var activeCell = e.parameter.activeCell;
var label = app.createLabel('file memorizzato correttamente');
app.add(label);
SpreadsheetApp.openById(activeSpreadsheet).getSheetByName(activeSheet).getRange(activeCell).setFormula(value);
app.close();
return app;
}

Answer 1

According to Your Question
Since yesterday all was gone fine, but since this morning i receive this generic error: https://docs.google.com Error encountered: An unexpected error occurred
According to Google developers page
So, I think problem may be in
var doc = DocsList.getFolderById('XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX').createFile(fileBlob);
try to use DriveApp instead of DocList. more info