Hello,
this issue is still present in mIRC 7.58.
Depending on how you press the two keys (which order), you can trigger this, I'm not so sure it's the same way as the original way described in this thread.
There is another problem, which may be related, if I let mIRC freeze until windows says 'not responding', then I can no longer break, at all (unless I type control + break from a different application).
I'm on windows 10 and my keyboard is a Logitech K120
