I tried to use
var findNotes = ENSession.SharedSession.FindNotes(noteSearch, null, ENSession.SearchScope.DefaultScope, ENSession.SortOrder.Normal, 100);
method as before, but it didn't response properly. Since today it isn't working for some reason. This method just response with 0 elements despite the value of noteSearch parameter.
I should mention that my sandbox key is full access not basic. And there is no single error in this response.
For some reason my previous request from the task still does not work:
var findNotes = ENSession.SharedSession.FindNotes(ENNoteSearch.NoteSearch("monday"), null, ENSession.SearchScope.DefaultScope, ENSession.SortOrder.Normal, 100);
But somehow works this:
var findNotes = ENSession.SharedSession.FindNotes(ENNoteSearch.NoteSearch(""), null, ENSession.SearchScope.Personal, ENSession.SortOrder.Normal, 21).Where(x => x.Title == "monday");
Anyway my problem have been solved.