Sorry for my late response.
i've found an article about the similar situation like me.
it suggested to insert some codes in util.cs.
(trying to insert BOM when the file is missing it)
i followed his words. but still got some weird charaters.
so i tried to insert the code(inserting BOM at the file header) before
i call htmleditor.LoadDocument() like this
byte preamble = UnicodeEncoding.Unicode.GetPreamble();
String byteOrderMark = UnicodeEncoding.Unicode.GetString(preamble, 0, preamble.Length);
HTMLFile = byteOrderMark + HTMLFile;
after that, it works fine so far.
i'm afraid i don't have much time to search why
just i hope some beginners like me can use this poor solution as a temporary one.