I was told that the code is invalid and can be used only once. Had to login using a backup code.
Oh, I didn’t know there is 2FA! 
@_MP
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I experienced this, too, using Google’s Authenticator app.
Switched to Microsoft’s Authenticator and things work smoothly now.
Is the 2FA required on every log-in, or is it once you’ve authenticated a computer, it doesn’t ask again? I wouldn’t mind registering once per computer, but wouldn’t want to do it each time I use the website.
It’s using TOTP, the app generating the code shouldn’t matter.
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Weird… Nothing changed on the forum (I mean, I make sure we’re always updated to its latest version). Please let me know via DM if you encounter the issue another time.
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For TOTP (this is what Google Authentificator uses) it is important that the time and date are correct.
Thanks for your advice.
Despite having worked for Google I didn’t want to insist on the Google Authenticator working for this* but was happy that the Microsoft Authenticator just worked when I first tried.
I attribute this non-discriminating attitude to getting older – I won’t insist on being right as long as it works. Both authenticators probably offer about the same level of security, even if I have only had exposure to an inside view at one of them. 
* Admittedly I disabled 2FA, de-registered and tried to re-register with the Google Authenticator app, but no luck.
I’ll say it felt a little odd since the Google Authenticator app had worked for years until it didn’t.
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TOTP should use the same algorithm, so I guess it is just a time issue. Can you verify that each app produces the same code? Maybe you can see if one generates with a delay.
Pay me for debugging this as you suggest and I’ll do it.
If not, I’ll waste my time instead on posting comments like this one on this forum.
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Well, if you photograph the QR code on both apps, you should see the code on both.