I did reproduce this, but it's not an issue with restarting mIRC. It's an issue with trying to load a new button background onto the toolbar after doing -fs.
Type: /toolbar -fs
Then, right click on it and choose Buttons > Select. Pick a button bar image (not individual icons, but an image to cover the entire thing) and try loading it. It will not load until you restart mIRC. I believe this can be shown with using the /toolbar command to load it as well, but this is the quickest way I can show it.
