In addition to the previous method, I found another new way to reproduce this bug on my computer.

On the desktop I right-click item "Personalization" ➔ "Change the color of borders, windows and taskbar".
As soon as I move the sliders to change the color, then the mIRC client breaks down and starts displaying the same bug described posts above.

I'm not sure it's only lack of RAM. It seems to me that this bug occurs when is used on the computer external or system applications, that affect all system windows and begins to interact with MDI windows, affecting their appearance during general modification and reconfiguration. As you correctly noted "Photoshop" also has its own unique window design, and it probably during launch automatically reconfigures its windows to this unique style by interacting with the system Windows, which theoretically somehow affects other open applications, including MDI windows that are used in the mIRC client and then it breaks them.

Most likely, be necessary to find some way to protect mIRC from any systemic negative impact that can disfigures an windows and their icons in mIRC client, and causes this bug.

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