I have not been able to reproduce this issue here so far, so my guess is that it is due to your anti-virus or firewall software. I am going to install Avast and Online Armor on a clean machine to see if I can reproduce the issue. Which versions of Avast and Online Armor are you using?

Also, if Avast or Online Armor have a "logging" or "logs" section, do they show any activity relating to mIRC?