I also am experiencing icue not working correctly with 3.21, it never shows up in systray but I can see it running in task manager. I've done everything that summerheat has done, but what did work for me was I copied the folder from x86 to the regular Program Files. It does run this way, but its annoying, doesn't auto start even if I tell it to in software, or modify the registry entires manually, and I still get all the same issues I get with 3.18 which are:
sometimes the color profiles don't load
sometimes forward and back buttons do not work
I often have to disconnect the USB receiver and reconnect it to get wireless to work
sometimes the mouse won't wake up from power saving/sleep
I've been messing with this for almost a month now, bouncing back and forth between 3.18 and 3.21 being dissatisfied with each version. It's a mouse, it should not be this complicated.
I've decided to return my Dark Core and Qi charging pad to Microcenter tomorrow and just pay the extra difference to get a product that actually works 100% from Brand X.