I had the same problem on Kmail/Linux. I found a workaround, but haven't tested it yet on Opera Mail on Windows.
I enabled an App password and set the authentication method to "clear text" instead of PLAIN.
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Same problem with another client (Kmail) on Linux (I'm also using 12.17 regularly on Linux, and Opera Mail on Windows btw, and I'm regularly discovering new functions). Just solved it. I'll check it on Windows and Opera Mail one of these days.
It worked fine for me without 2-step verification until a few weeks ago. Enabling it did the trick.
However the auth method also got changed from "Plain" to "clear text" in the Kmail settings by the automatic detection, dunno if it played a role.