Was struggling and messing with iCUE 4/5 because I needed to upgrade from iCUE 3 (since my headset wasn't being detected) and came across a small fix that might help if you've done everything and anything from clean installs to upgrades to messing with hardware.
If you're experiencing crashes on start, etc. one thing that might help is changing the compatibility mode. Make sure iCUE is closed before doing anything here.
1. Go to where your iCUE Launcher is installed (for me this was C:\Program Files\Corsair\CORSAIR iCUE 4 Software)
2. Right click on the file, go to properties, compatibility
3. Check the box that says "Run this program in compatibility mode for" and set it to another Windows version (preferably no older than 7)
4. Relaunch iCUE (might need to do it as an admin) and it should just work.
For me I had to set it to Windows 7 (I'm running Windows 10) and it fixed all the issues.
