I've been able to reproduce and track down the bug to a certain extent. It's probably in wine, but multiple workarounds are possible (see below). I'm posting this mostly for the other wine users out there and because google brings up this thread.
The real bug:
When you CTRL+Minimize the mirc window, it will ask to set a password (unless one is set), and this dialog as well as the one for entering the password for unlocking is buggy on wine. (These dialogs do not accept input at all under these circumstances.) Mirc happily runs in the background, though, afaik.
The problem occurs on linux window managers when you use ALT+CTRL+LEFT/RIGHT to switch desktops ... That seems to send a minimize signal to the mirc window while CTRL is down. So, it asks for a password, and things break.
There are many possible Workarounds:
* Use mirc in Fullscreen mode (F11). That seems to avoid the minimization event.
* Run mirc in a wine Virtual Desktop (use winecfg to enable that).
* Change the shortcuts for the WM
Sadly, changing how mirc reacts to CTRL+minimize does not seem to be a configurable option. Nor can one disable the locking mechanism entirely. Setting a password does not solve the problem either, as the unlocking dialog hangs in the same way without input.
Interestingly, the password dialogs seem to work fine when activated through the Lock/Unlock-Button in the last Options menu. Just the combination of being minimized and the dialog popping up seems to screw with wine.
wine-1.3.15 (Ubuntu 11.04, Gnome 2.x)
Well, I guess I now have to register at winehq as well ...