Update:
I tried several things, nothing really worked for me. The flashes kept appearing.
I came back to an old Idea, which I tried for the DX Crashes. That they might be caused by Power Spikes, created when the GPU tries to pull more Power from the PSU, but sometimes something goes wrong and they cause a crash.
Back then, it did not help with the crashes, and the Idea went into the next corner.
But I came to the Idea, that those spikes could cause the flashes to appear. And it worked for me.
I guess, since the Fall Update, Win10 tries to lower the CPU Power as soon as it thinks, the Software is running in the background and this causes Power Spikes, which create those weird flashes.
(Or like in my case, it changed it back to the old Default settings)

Control Panel>System and Security>Power Options> Change Plan Settings>Change advanced power settings> Find PCI Express > Link Power State Management > and set to OFF.
This is, what I changed and the flashes disappeared for me. Ymmv.