"Your request status has changed to Done with resolution Known Error."
Truly unbelievable.

I just filled a bug report with this info, hopefully it will be helpful to opera devs... I will share any update on the subject. Hope it's fixed soon!

I have the very same problem, Opera 77.0.4054.146 on Linux Mint Cinnamon 20.1. Dark Reader extension.
In my case, the problem is easy to reproduce: just type about:blank and Bam, white flash. The same thing happens with about:gpu, but that's another story.
Please notice that about:blank is pretty common in web browsing, some websites trigger them in anchors with target:"_blank" or with pure javascript (window.open). The second method seems not to be affected with the bug.
I can reproduce the issue with a simple html file containing something like:

<a href="https://google.com" target="_blank">Open site in another tab (target attr)</a><br>

Hope this helps anyone!