i played around with some settings.
What is did:
This already decreased the problem.
Then, I disabled my (only) plugin:
'Ublock Origin' >> instantly the flickering and content mixing between Opera windows is gone, NICE!
Next, I tried disabling the build-in spam blocker to check if these two were conflicting with each other. but that was not the case.
Will do some testing later, as my current (second) monitor setup will not exhibit this odd behavior.