User cannot log in with 2-factor auth. Says "each code can only be used once"

The user was connected with 2-factor auth before. Suddenly he was denied login (after connecting from another device). Trying to solve this i as admin deleted his old account (it was new, no replies yet so we didn’t mind), but the problem repeats when he tried to create a new account:


the user deleted the account from his google authenticator app, and re-scanned the new QR code. that didn’t help.
what “code” is being used again?

Discourse version: 9da9b2e1

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Looking for clues for this error message:

It happened still with me when I was creating new account on Unix Linux Community Forum. I think the 2-Auth there is bugged and not supported