You could pull the preferences from an backup, when it was working.
Failing in having a pre-failure backup copy, you might try this: Open your Preferences menu and screenshot each menu so you have a visual record of your customizations (there are other menus that have settings besides Preferences…so screenshoot them as well). Once done, make a copy of your current Preferences folder and move it to your desktop. Delete the original folder and launch Cubase. If the problem goes away, start dragging files from the copied folder back into the original folder and reboot Cubase with each addition until the problem occurs again. Then you’ll know which pref was corrupted. At that point, you can delete the Preference folder and drag everything _but_the offending pref from the copied folder into the original and have _most_of your prefs restored. Yes, an annoyingly lengthy and tedious process.
The less tedious variation to this is to delete the original prefs folder and launch. if the problem is gone, refer to the screenshots and reset the prefs from them.