Thanks for your bug report. I have not yet been able to reproduce this issue by following the steps you described.
If you can please post the output of the //echo command in westor's post, that will help in tracking down the issue.
Also, if you could please try this:
1) Run the mIRC v7.43 installer
2) Choose an mIRC install folder in your Documents folder.
3) Check the "portable" option in the installer.
4) Install mIRC.
Now run mIRC from that folder. Are you still able to reproduce the issue with this copy of mIRC?