Both latest Opera Stable and Opera Developer crash when trying to run with "New private window" only if the main browser is not running.
I am using Opera browser for at least 10 years constantly updating it. For about 4 days I've noticed I can't open "New private window" when the main browser is not running.
After a couple of hours trying to figure out what's cause of the issue I've noticed when disabling "Sync data" the problem disappears.
Going further with the investigation I concluded that the root of the issue is "Settings" in Sync data that is causing crashing both browser versions. All others can be left checked. Good luck!