To fix the CUBE GUI flickering:
Locate your CUBE application folder. You can find it here:
Windows
{USER}\AppData\Roaming\Lunacy Audio\CUBE
Mac
Users/{USERNAME}/Library/Application Support/Lunacy Audio/CUBE
Open the GeneralSettings.xml and change the OPEN_GL="1" to OPEN_GL="0" (or vice versa depending on your current settings)
Relaunch CUBE