@zeej Thank you for the solution. It is now 6 months later (Nov '21) and it just happened with me with the latest latest release - clearly the problem is not resolved, but thank you for your workaround!
In my case, if you hover over the taskbar icon, it shows is a small preview window with all the screen detail of the actual Opera session window (it is not blank), but it will simply not display on either my screen or HDMI Monitor. The screen is therefore generated, but not displayed.