No, I didn’t - so thank you for that. I had helped myself by creating a new admin user and logging in as this user and using the web interface to disable 2FA for the affected users.
I have now followed your link (thank you for that) and it worked insofar that my 2FA with a YubiKey has been disabled.
But using 2FA with OTP again? No. I can’t add an authenticator app, because after scanning the QR-Code, the generated token is not valid. Which was NOT a problem on the original instance.