I've had this a fair few times over the past months and the only fix was to reboot the PC.
Finally I tried something else. In Windows Task Manager, when Cube IDE is not running, you should not see any of these
If you do, terminate that process.
That saves the PC reboot.
Clearly the software leaves a bit of itself running and that prevents it working. I have others which do the same e.g. the Vegas Pro 16 video editor which needs this every time you run it...
This may help somebody.