I have the exact same problem. (oddly enough with the same character set).

My post is in this thread

To me, the problem seams that when the Enter key is hit it is getting converted to single byte characters.

As you type it the text displays as unicode - but as soon as what you typed gets removed from the text box (hitting enter/ctrl+up-down) it gets replaced with single byte characters "?"

I have also tried the Greek & Hebrew character sets (which i also have fonts for) and have the same problem.

I found it works fine in mIRC 5.91 - None of the suggestions in this thread have helped at all.