I am sorry to hear you're having this issue. It might be a good start to check if your computer meets the system requirements for Archicad 19. You can check them here.
If you can, please share more details about your machine. Are you using Windows 10? Have you experienced the flickering in a specific window or the whole screen? If everything on the screen is flashing, you might want to check this troubleshoot from Microsoft Support.
Now just out of curiosity, why did you choose to downgrade from AC22 to AC19?
I hope I could help you somehow. Let us know how it goes.
Customer Engagement Expert | GRAPHISOFT SE
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