Update : Answer from Apple Developer Support
Seems that Xcode 11.3.1 isn't compatible with macOS Big Sur, have to roll back to Catalina for developper account integration.
Guitool
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Hello,
I've the same problem : macOS Big Sur, Xcode 12.4 and Xcode 11.3.1; can't login with my 2 factors account on 11.3.1.
I've tried original password and app generated password, without any success.
After successfully login within v12.4, dev account is listed on v11.3.1 prefs but appear disconnected and I've the "Unknown error occurred" when entering password.
It's also critical for me.
Thanks for any ideas